Human Health and Diseases | ISC Class 12 Biology Notes

This note covers health, communicable diseases, their causes and prevention, the malaria parasite's life cycle, innate and acquired immunity, antibodies, vaccination, lymphoid organs, allergies, autoimmunity, human immunodeficiency virus infection, cancer, smoking, and drug and alcohol abuse.

What do health, disease and infection mean?

Definition: Health is a state of complete physical, mental and social well-being. It involves more than physical fitness or the absence of disease.

A disease occurs when normal functioning of one or more organs or body systems is disturbed, producing signs and symptoms. Health is influenced by inherited defects, infections and lifestyle, including food, water, rest, exercise and habits.

The mind can influence the body's defences through the nervous and hormone-producing systems. Balanced nutrition, personal cleanliness and regular exercise support health. Waste disposal, clean food and water, and control of disease-transmitting organisms protect community health.

How are disease-causing agents classified?

Pathogens are disease-causing agents. A parasite lives in or on another organism, its host, and obtains resources from it. Most parasites are pathogens because they harm their hosts. Pathogens enter the body, multiply and interfere with normal activities.

Communicable diseases can pass from an infected person to another person, directly or indirectly. Non-infectious conditions, such as cancer, involving uncontrolled cell growth, are a different category. Identify both the causative agent and its route of transmission: the organism causing a disease need not be the organism carrying it.

How do the bacterial diseases spread and affect the body?

What distinguishes typhoid from pneumonia?

Typhoid is caused by Salmonella typhi. The bacteria generally enter the small intestine through contaminated food or water and travel through blood to other organs. Common symptoms include sustained high fever of 39 to 40 degrees Celsius, weakness, stomach pain, constipation, headache and loss of appetite.

In severe cases, intestinal perforation, meaning a hole in the intestinal wall, and death may occur. The Widal test is a blood test used in typhoid diagnosis. Safe drinking water, clean food and hygienic disposal of excreta help interrupt transmission.

Pneumonia can be caused by Streptococcus pneumoniae and Haemophilus influenzae. It affects the alveoli, the air sacs of the lungs. Fluid accumulating in these sacs makes respiration difficult. Fever, chills, cough and headache are symptoms; in severe cases, lips and fingernails may become grey to bluish.

Infection can spread through inhaled droplets from an infected person or by sharing glasses and utensils. Avoiding close contact with infected persons and their belongings helps prevent transmission. Unlike pneumonia, the common cold affects the nose and respiratory passages but not the lungs.

What are diphtheria and plague?

Diphtheria is caused by Corynebacterium diphtheriae. It spreads mainly through respiratory droplets. Fever and sore throat can accompany a thick membrane in the throat that may obstruct breathing. Vaccination, introducing a preparation that stimulates protective immunity, and avoiding exposure to infectious droplets are preventive measures.

Plague is caused by Yersinia pestis. In bubonic plague, infected fleas associated with rodents transmit the bacteria. Fever and painful, swollen lymph nodes, called buboes, are characteristic. Lymph nodes are small immune-system structures distributed along vessels carrying lymph, a tissue-derived fluid.

Plague prevention includes flea control, rodent control and avoiding contact with infected animals. Pneumonic plague, which involves the lungs, can spread through respiratory droplets. Thus, describing all plague transmission as flea-borne would omit an important route.

How do common cold, dengue and chikungunya differ?

These are viral diseases, but they do not share one transmission route. Rhinoviruses causing common cold spread through droplets and contaminated objects. Dengue and chikungunya are transmitted by infected Aedes mosquitoes. A vector is an organism that transmits a pathogen between hosts.

Common cold symptoms usually last for 3 to 7 days. Coughing and sneezing release infectious droplets. Pens, books, cups, doorknobs and computer equipment can become contaminated. Avoiding close contact with infected persons and their belongings helps reduce transmission.

How does vector control prevent disease?

Mosquito control includes preventing stagnant water around homes, regularly cleaning household coolers, and using mosquito nets and wire mesh on doors and windows. Fish such as Gambusia feed on mosquito larvae, the immature stages, and can help control them in ponds.

Controlling the mosquito and its breeding places interrupts the movement of the virus between people. The mosquito is the carrier; it is not the virus causing dengue or chikungunya. Similarly, a female Anopheles mosquito transmits the protozoan responsible for malaria.

How does Plasmodium complete its life cycle?

Malaria is caused by species of Plasmodium, a protozoan parasite. Plasmodium falciparum causes the most serious form, malignant malaria, which can even be fatal. The parasite requires two hosts, a human and a mosquito, to complete its life cycle.

The transmitting vector is an infected female Anopheles mosquito. Sporozoites are the parasite's infectious forms introduced into humans during its bite. Red blood cells, abbreviated RBCs, are blood cells that transport oxygen and are attacked during the human phase.

What is the sequence in humans and mosquitoes?

1. An infected female mosquito bites a human and introduces sporozoites. They reach the liver through the blood.
2. The parasites reproduce asexually, meaning without fusion of sex cells, in liver cells. The cells burst and release parasites into the blood.
3. The parasites enter RBCs and multiply. Rupture of these cells releases parasites that can infect further RBCs.
4. RBC rupture releases the toxic substance haemozoin, associated with chills and high fever recurring every three to four days.
5. Sexual stages called gametocytes develop in RBCs. Another female mosquito takes them up with a blood meal.
6. Fertilisation and further development occur in the mosquito's gut. Infective sporozoites eventually migrate to its salivary glands, ready for transmission during another bite.

The diagram separates human and mosquito hosts. Arrows connect the mosquito bite, liver cells, red blood cells, male and female gametocytes, mosquito gut and salivary glands. It shows asexual reproduction in humans and fertilisation in the mosquito.

See Fig. 7.1 in your NCERT textbook

For a graphic outline, keep the two hosts distinct and show the direction of transfer. Sporozoites enter the human; gametocytes enter the mosquito. Mosquito control, nets and prevention of stagnant water act on transmission rather than on multiplication already occurring inside human cells.

What causes amoebiasis, worm infections and ringworm?

How are intestinal infections acquired?

Amoebiasis, also called amoebic dysentery, is caused by Entamoeba histolytica, a protozoan parasite of the large intestine. Symptoms include constipation, abdominal pain and cramps, and stools containing excess mucus and blood clots.

Houseflies serve as mechanical carriers: they transfer the parasite from infected faeces to food. Food and water contaminated with faecal matter are the main sources of infection. Clean drinking water, safe food and proper disposal of excreta are therefore important preventive measures.

Ascariasis is caused by the intestinal roundworm Ascaris. Eggs leave an infected person in faeces and contaminate soil, water and plants. Infection is acquired through contaminated water, vegetables or fruits. Symptoms include internal bleeding, muscular pain, fever, intestinal blockage and anaemia, reduced oxygen-carrying capacity of blood.

How do filariasis and ringworm differ?

Filariasis, or elephantiasis, is caused by filarial worms such as Wuchereria bancrofti. Female mosquito vectors transmit the pathogens. The worms cause slowly developing, long-lasting inflammation, usually in the lymphatic vessels of the lower limbs. These vessels carry lymph. Genital organs are also often affected, producing gross deformities.

Ringworm is a fungal infection, despite its name. Microsporum, Trichophyton and Epidermophyton can cause it. Dry, scaly affected patches on the skin, nails or scalp are accompanied by intense itching. Heat and moisture favour fungal growth, particularly in skin folds and between toes.

Ringworm is generally acquired from soil or shared towels, clothes and combs of infected individuals. Keeping the body clean and avoiding contaminated personal items help prevent it. Control of mosquitoes is relevant to filariasis, whereas sanitation interrupts the faecal contamination responsible for amoebiasis and ascariasis.

How does innate immunity provide immediate barriers?

Definition: Immunity is the body's overall ability to fight disease-causing organisms through its immune system. Innate immunity is the non-specific defence present from birth.

Non-specific means that the defence is not directed against one particular pathogen. Innate immunity supplies barriers that prevent entry, inhibit microbial growth or act against agents that enter. Its four barrier categories must be distinguished by their functions.

What do the cellular and cytokine barriers do?

Leukocytes are white blood cells involved in defence. Neutrophils and monocytes are examples. Macrophages are tissue cells that engulf and destroy microbes. This engulfment is called phagocytosis. Natural killer cells are defensive lymphocytes, a type of white blood cell, that kill infected cells.

Cytokines are proteins involved in communication between cells. Interferons are protective proteins secreted by virus-infected cells. Their source is the infected cell, while their protective effect operates on cells that are not yet infected. They differ from antibodies, proteins that bind specific antigens, substances recognised by the immune system that can provoke a specific response.

The skin and mucus belong to the physical category, whereas acid, saliva and tears belong to the physiological category. This classification separates a structure that blocks or traps microbes from conditions or secretions that hinder their growth.

How do acquired immunity and antibodies work?

Acquired immunity is pathogen-specific and characterised by memory. An antigen is a substance recognised by the immune system that can provoke a specific response. The first encounter produces a low-intensity primary response; subsequent encounter with the same pathogen produces a highly intensified secondary response.

How do humoral and cell-mediated responses differ?

B-lymphocytes produce antibodies, proteins that bind specific antigens. Their response is called humoral immunity because antibodies occur in body fluids, including blood. T-lymphocytes participate in cell-mediated immunity; T-cells themselves do not secrete antibodies, though they help B-cells produce them.

Cell-mediated immunity is responsible for graft rejection, the immune attack on transplanted tissue recognised as foreign. Tissue and blood-group matching are important before transplantation. The distinction between the body's own components, or self, and foreign components, or non-self, underlies these responses.

What is the structure of a typical antibody?

An antibody has four peptide chains, chains of amino acids forming a protein: two long heavy chains and two shorter light chains. It is represented as H₂L₂, where H means heavy chain, L means light chain, and each subscript 2 gives the number of that chain.

The cartoon shows a Y-shaped molecule with two longer inner heavy chains and two shorter outer light chains. Antigen-binding sites are labelled at both arm tips. Links join the chains to one another.

See Fig. 7.4 in your NCERT textbook

Where do the five antibody classes occur?

Immunoglobulin, abbreviated Ig, means antibody. G, A, M, D and E identify its classes. Serum is the fluid remaining after blood clots; colostrum is the first yellowish milk. The placenta connects maternal and foetal exchange; mast cells are immune cells that release allergy-related chemicals.

A receptor is a cellular structure to which a particular chemical binds. Antigen receptors recognise antigens, allowing cells to respond to their presence.

Antibody class, site of occurrence and function are related. Identify the class, where it occurs and how it contributes to protection.

How do active immunity, passive immunity and vaccination differ?

Active immunity develops when exposure to antigens stimulates the host to produce antibodies. It can follow natural infection or vaccination. It is slow and takes time to reach its full effective response. Passive immunity results when ready-made antibodies are supplied to the recipient.

Antibodies in maternal colostrum protect the newborn, and antibodies pass from mother to foetus through the placenta, the organ connecting maternal and foetal exchange. Both are examples of passive immunity because the antibodies originate outside the recipient's body.

How does vaccination use immune memory?

A vaccine is a preparation containing antigenic proteins or inactivated or weakened pathogens that stimulates protective immunity. Immunisation means establishing immune protection; it can be active through vaccination or passive through administration of preformed antibodies.

1. A vaccine introduces pathogen-related antigens into the body without requiring the person to undergo the corresponding natural infection.
2. The immune system recognises the introduced antigens and mounts a specific response, including production of antibodies.
3. Memory B-cells and T-cells are generated. These cells retain the capacity to respond to a later encounter with the same pathogen.
4. On subsequent exposure, the pathogen is recognised quickly and a rapid, intensified response helps overwhelm it.

For protection requiring a quick response, preformed antibodies may be supplied. An antitoxin is a preparation containing antibodies against a toxin, a harmful substance. Tetanus antitoxin and antibodies against snake venom illustrate passive immunisation.

Note: A vaccine stimulates the recipient's response; passive immunisation supplies antibodies already made elsewhere. Receiving maternal antibodies does not mean that the infant produced those antibodies after vaccination.

Recombinant technology, which combines genetic material from different sources, enables production of pathogen proteins in bacteria or yeast. The hepatitis B vaccine can be produced using yeast. This method supports large-scale vaccine production and availability.

How do lymphoid organs, allergy and autoimmunity relate to defence?

Where do lymphocytes develop and respond?

Lymphoid organs are sites of origin, maturation or multiplication of lymphocytes. Primary lymphoid organs, bone marrow and thymus, support the development of immature lymphocytes into antigen-sensitive cells. Bone marrow is the main site of blood-cell production.

The thymus is a lobed organ near the heart beneath the breastbone. It is quite large at birth, decreases with age and becomes very small by puberty. Mature lymphocytes move to secondary lymphoid organs, where they encounter antigens and multiply into cells that carry out immune responses.

Secondary sites include the spleen, lymph nodes, tonsils, Peyer's patches in the small intestine and appendix. The spleen filters blood, trapping blood-borne microbes, and contains a large reservoir of RBCs. Lymph nodes trap microbes and other antigens entering lymph and tissue fluid.

Mucosa-associated lymphoid tissue, abbreviated MALT, occurs in the lining of respiratory, digestive and urogenital tracts. It constitutes about 50 per cent of the body's lymphoid tissue. This places defensive tissue at surfaces through which foreign agents can enter.

A human outline contains a branching network of lymphatic vessels. The labelled structures are lymph nodes, thymus and lymphatic vessels. The drawing shows their distribution within the body.

See Fig. 7.5 in your NCERT textbook

How are allergy and autoimmunity different?

Allergy is an exaggerated immune response to certain environmental antigens. The substances provoking it are allergens, such as pollen, dust mites and animal dander, the small flakes shed from animal skin. IgE antibodies are involved in these responses.

Release of chemicals such as histamine and serotonin from mast cells contributes to allergy. Symptoms include sneezing, watery eyes, a running nose and difficulty breathing. Antihistamines, which oppose histamine's effects, and other medicines can reduce symptoms.

Autoimmunity occurs when the immune system attacks self-cells and damages the body. Sometimes genetic and other unknown reasons contribute to this failure of self-recognition. Rheumatoid arthritis, a disease affecting joints, is an example. Allergy targets environmental allergens; autoimmunity targets the body's own components.

How does HIV cause AIDS, and how can transmission be prevented?

Human immunodeficiency virus, or HIV, causes acquired immunodeficiency syndrome, or AIDS. A syndrome is a group of symptoms; immunodeficiency means weakened immune defence. HIV is a retrovirus, which copies its genetic material from ribonucleic acid (RNA) into deoxyribonucleic acid (DNA) inside a host cell.

RNA and DNA carry genetic information. HIV has an envelope surrounding its RNA genome, the complete viral genetic material. Its enzyme reverse transcriptase produces a DNA copy using viral RNA as a template.

What happens in an infected cell?

1. HIV enters a macrophage and introduces its RNA. Reverse transcriptase produces viral DNA from this RNA.
2. Viral DNA becomes incorporated into the host cell's DNA and directs production of new viral material.
3. The infected cell produces new viruses. Macrophages can continue producing virus particles rather than being immediately destroyed.
4. HIV also enters helper T-lymphocytes, immune cells that help coordinate immune responses, and produces further viruses.
5. New viruses infect other helper T-cells. Their number progressively decreases, reducing the person's ability to overcome infections.

The plasma membrane is the cell's outer boundary; the nucleus contains its genetic material. Cytoplasm is the interior material outside the nucleus.

Arrows show viral RNA entering an animal cell, viral DNA being produced by reverse transcriptase, incorporation into the host genome, new viral RNA production and release of new viruses. The labelled cell includes its plasma membrane, cytoplasm and nucleus.

See Fig. 7.6 in your NCERT textbook

An infected cell can survive while viruses are replicated and released.

What are the symptoms, diagnosis and transmission routes?

Fever, diarrhoea and weight loss may occur as infection progresses. Loss of helper T-cells permits infections that the body could otherwise overcome. Enzyme-linked immunosorbent assay, or ELISA, is an antigen-antibody-based diagnostic test used in investigating HIV infection.

HIV infection generally spreads through sexual contact with an infected person, contaminated blood or blood products, sharing infected needles, and transmission from an infected mother to her child through the placenta. It is not spread by mere touch or ordinary physical contact.

Prevention includes screened blood, disposable needles and syringes, avoiding shared injection equipment, condom use and safer sexual behaviour. People living with HIV need support and care. Ordinary social contact is not a reason to isolate them from family or society.

How do cancer cells arise, spread and respond to treatment?

Cancer involves a breakdown in the regulation of cell growth and differentiation, the process by which cells acquire specialised features. Contact inhibition normally restrains uncontrolled cell growth when cells touch neighbouring cells. Cancer cells appear to have lost this property and continue dividing into masses called tumours.

How do benign and malignant tumours differ?

Metastasis is the spread of cells from a malignant tumour to distant sites, where they establish new tumours. Cells detached from the original mass can travel through blood. Rapidly growing tumour cells also compete with normal cells for vital nutrients.

What causes cancer and how is it detected?

Carcinogens are physical, chemical or biological agents capable of inducing cancerous change. Examples include X-rays, gamma rays, ultraviolet radiation and chemicals in tobacco smoke. Radiation can damage DNA; tobacco-smoke carcinogens are a major cause of lung cancer.

Oncogenic viruses are cancer-causing viruses carrying cancer-promoting genes. Proto-oncogenes are genes in normal cells that, when activated under certain conditions, could lead to cancerous transformation. Their presence alone does not mean a normal cell is already cancerous.

A biopsy removes a piece of suspected tissue for examination. Histopathology studies disease-related tissue changes microscopically after thin sections are prepared and stained. Blood and bone marrow tests can detect increased cell counts in leukaemias, cancers of blood-forming tissues.

Computed tomography, or CT, uses X-rays to generate three-dimensional images. Magnetic resonance imaging, or MRI, uses strong magnetic fields and non-ionising radiation. These techniques help detect internal cancers. Early detection allows successful treatment in many cases.

What are the main treatment approaches?

Surgery removes tumour tissue. Radiotherapy uses radiation to kill tumour cells while protecting surrounding normal tissue. Chemotherapy uses drugs against cancer cells; most cancers are treated through combinations of surgery, radiotherapy and chemotherapy. The majority of these drugs have side effects such as hair loss and anaemia.

Immunotherapy helps the immune system attack cancer. Biological response modifiers, substances that alter immune responses, include alpha-interferon, which activates immune defence and helps destroy tumours. Treatment choice is distinct from diagnosis: imaging locates abnormalities, whereas these approaches act against tumour tissue.

What are the sources and effects of commonly abused drugs?

Drug abuse includes taking drugs for non-medicinal purposes or in quantities or frequencies that impair physical or psychological functioning. Some drugs have legitimate medical uses, but this does not make their misuse harmless. Morphine, for example, is an effective painkiller used after surgery.

The central nervous system comprises the brain and spinal cord. The cardiovascular system consists of the heart and blood vessels. Dopamine is a neurotransmitter, a chemical carrying signals between nerve cells. Sedation means reduced alertness and activity.

How do the principal drug groups compare?

A depressant reduces activity; a stimulant increases it. These contrasting actions help distinguish heroin from cocaine.

Heroin, also called smack, is chemically diacetylmorphine, derived by chemical modification of morphine. It is generally taken by snorting or injection. Its depressant effect slows body functions. Opioid receptors occur in the central nervous system and gastrointestinal, or digestive, tract.

Cannabis flower tops, leaves and resin are used in combinations producing marijuana, hashish, charas and ganja. These preparations are generally inhaled or swallowed. Natural cannabinoids interact with receptors present principally in the brain, rather than being described as acting exclusively there.

Cocaine is usually snorted and produces euphoria, an intense feeling of well-being, and increased energy. Excessive dosage causes hallucinations, perceptions without a corresponding external stimulus.

Barbiturates are manufactured drugs, not plant extracts. Their sedative effect means that they reduce alertness and activity; their sleep-producing action is called hypnotic. They are among medicinal drugs that are often abused. Their source and effects should be distinguished from those of plant-derived opioids and cocaine.

Why do addiction, alcohol abuse and smoking threaten health?

How do experimentation and repeated use become harmful?

Adolescence is the transition from childhood towards adult maturity. Curiosity, experimentation, a desire for excitement, stress, peer pressure and an unsupportive family environment can encourage substance use. Perceived relief or social acceptance can lead to repeated use.

Addiction is psychological attachment to effects such as euphoria or temporary well-being. Repeated use increases tolerance, so higher doses are needed to produce the response. Even one use can be a forerunner to addiction; it does not follow that addiction inevitably occurs after one exposure.

Dependence is the body's tendency to develop an unpleasant withdrawal syndrome when regular use abruptly stops. Symptoms include anxiety, shakiness, nausea and sweating. In some cases withdrawal can be severe and even life-threatening, requiring medical supervision.

What damage can alcohol, drugs and tobacco cause?

Excessive drug doses may cause coma or death from respiratory failure, heart failure or bleeding in the brain. Combining drugs or taking them with alcohol generally results in overdosing and can cause death. Sharing infected needles also transmits serious infections, including HIV.

Chronic drug and alcohol use damages the nervous system and liver. Cirrhosis is scarring of the liver associated with chronic injury. Use during pregnancy adversely affects the foetus. Alcohol use in adolescence may have long-term effects and could lead to heavy drinking in adulthood.

Nicotine, an alkaloid or nitrogen-containing organic compound in tobacco, stimulates the adrenal glands, hormone-producing glands above the kidneys. They release adrenaline and noradrenaline, hormones that raise blood pressure and heart rate. Smoking is associated with increased incidence of lung, bladder and throat cancers, heart disease, bronchitis, inflammation of the airways, and emphysema, damage to alveolar walls.

Tobacco chewing is associated with increased risk of oral-cavity cancer. Smoking increases carbon monoxide, a gas, in blood and reduces oxygen bound to haemoglobin, the oxygen-carrying pigment. This produces oxygen deficiency. Tobacco use also has an addictive nature.

How can abuse be prevented and controlled?

• Avoid undue pressure: respect individual abilities and choices rather than pushing beyond a person's limits.
• Provide education and counselling: help adolescents face problems and disappointment, and encourage healthy pursuits such as sport, music and reading.
• Seek support: parents, trusted friends and peers can help a person discuss anxiety, guilt and difficult situations.
• Recognise warning signs: falling academic performance, isolation, aggression, neglected hygiene and changing sleep or eating habits need attention.
• Obtain professional help: psychologists, psychiatrists and rehabilitation programmes support treatment and recovery.

Effects extend beyond the individual to family relationships, mental distress and financial strain. Early support and professional care are therefore important alongside education about biological harm. Warning signs should prompt help and assessment, rather than being treated as proof of a particular substance being used.

Glossary

• Pathogen — A disease-causing agent that can interfere with normal functioning after entering a host.
• Vector — An organism that carries and transmits a pathogen between hosts.
• Sporozoite — The infectious stage of Plasmodium introduced into humans by an infected mosquito.
• Antigen — A substance recognised by the immune system that can provoke a specific response.
• Antibody — An immune protein produced by B-lymphocytes that binds a specific antigen.
• Innate immunity — Non-specific defence present from birth, involving physical, physiological, cellular and cytokine barriers.
• Acquired immunity — Pathogen-specific defence characterised by memory and a stronger response after subsequent exposure.
• Passive immunity — Protection obtained through antibodies supplied from outside the recipient's own immune response.
• Allergen — An environmental antigen that provokes an exaggerated immune response in a sensitive person.
• Autoimmunity — A condition in which immune responses attack the body's own cells and cause damage.
• Metastasis — Spread of malignant tumour cells to distant sites where they establish new tumours.
• Carcinogen — A physical, chemical or biological agent capable of inducing cancerous transformation in cells.
• Addiction — Psychological attachment to particular effects of drugs or alcohol that encourages repeated use.
• Dependence — The tendency to develop unpleasant withdrawal symptoms when regular substance use abruptly stops.

Common errors and misconceptions

• Misconception: Every disease is communicable. Correct: Diseases include infectious and non-infectious categories. Cancer is a non-infectious condition involving disrupted regulation of cell growth, whereas typhoid spreads through contaminated food and water.
• Misconception: A mosquito is the pathogen causing malaria. Correct: Plasmodium is the protozoan pathogen. An infected female Anopheles mosquito acts as the vector and is also a host in its life cycle.
• Misconception: Ringworm is caused by a worm. Correct: Ringworm is fungal. Ascariasis and filariasis are examples of helminth infections; similar-sounding names do not establish the pathogen's biological group.
• Misconception: All immune cells produce antibodies. Correct: B-lymphocytes produce antibodies. T-lymphocytes mediate cellular responses and can help B-cells, but do not themselves secrete antibodies.
• Misconception: Colostrum gives active immunity. Correct: Its preformed maternal antibodies provide passive immunity to the infant. Active immunity involves the recipient producing a response after antigen exposure.
• Misconception: HIV spreads by touching an infected person. Correct: Mere touch and ordinary physical contact do not transmit HIV. Relevant routes involve sexual contact, contaminated blood, infected needles and mother-to-child transmission.
• Misconception: A benign tumour is the same as a metastatic tumour. Correct: Benign tumours normally remain localised. Metastasis describes malignant cells spreading and establishing growths at distant sites.
• Misconception: Addiction and dependence mean exactly the same thing. Correct: Addiction concerns psychological attachment to effects. Dependence concerns withdrawal symptoms when regular use stops; both can sustain harmful repeated use.

Exam-style questions with model answers

Q1. Define a pathogen and a vector. Illustrate each using malaria. [2 marks]

1. A pathogen causes disease. In malaria, the pathogen is the protozoan parasite Plasmodium.
2. A vector transmits a pathogen between hosts. An infected female Anopheles mosquito is the malaria vector.

Q2. For typhoid, state its causative bacterium and transmission route, give two common symptoms, and name the diagnostic test. [3 marks]

1. Typhoid is caused by Salmonella typhi. It generally enters the small intestine through contaminated food or water and can spread to other organs through blood.
2. Two common symptoms are sustained high fever and stomach pain. Weakness, constipation, headache and loss of appetite are other possible symptoms.
3. The Widal test is used in the diagnosis of typhoid.

Q3. Explain the four barrier categories of innate immunity, giving at least one example and its protective role for each. [4 marks]

1. Physical barriers obstruct entry or trap microbes. Skin blocks entry, while mucus lining body passages traps microorganisms before they penetrate deeper tissues.
2. Physiological barriers inhibit microbial growth. Stomach acid, saliva and tears provide examples of these protective conditions or secretions.
3. Cellular barriers include defensive white blood cells. Neutrophils and macrophages engulf and destroy microbes through phagocytosis.
4. Cytokine barriers include interferons. Virus-infected cells release these proteins, which protect non-infected cells against further viral infection.

Q4. Outline the life cycle of Plasmodium in five stages, starting with entry into a human and ending with the next infectious mosquito. Include the stages transferred between hosts. [5 marks]

1. An infected female Anopheles mosquito introduces sporozoites during a bite. These are the infectious forms entering the human, and they reach liver cells through blood.
2. The parasite multiplies asexually in liver cells. When these cells burst, parasites are released and subsequently attack red blood cells.
3. Repeated multiplication ruptures red blood cells. Released parasites infect further cells, while haemozoin released during rupture is associated with chills and fever.
4. Sexual stages called gametocytes develop in red blood cells. A female mosquito takes them into its body while feeding on an infected person's blood.
5. Fertilisation and development occur in the mosquito's gut. Sporozoites migrate to the salivary glands and can enter another human during a subsequent bite.

Q5. Explain in five points how HIV replication leads to reduced immune defence. Include the roles of reverse transcriptase, host DNA and helper T-cells. [5 marks]

1. HIV enters macrophages and releases its RNA genetic material. The viral enzyme reverse transcriptase makes a DNA copy from this RNA template.
2. The viral DNA becomes incorporated into host-cell DNA. It directs the infected cell to produce material needed for new virus particles.
3. Macrophages continue producing viruses and act as a source of infection. HIV also enters helper T-lymphocytes and replicates within these immune cells.
4. Newly produced viruses infect additional helper T-cells. Repetition of this process progressively reduces the number of helper T-cells in the body.
5. Reduced helper T-cell numbers weaken immune defence. The person becomes susceptible to infections that could otherwise have been overcome, producing the immunodeficiency associated with AIDS.

Q6. Explain how vaccination produces active immunity, why its full protection takes time, how immune memory helps later, and how passive immunisation differs. [4 marks]

1. Vaccination introduces pathogen-related antigens, such as antigenic proteins or weakened or inactivated pathogens. These stimulate the recipient's own immune response and antibody production.
2. Active immunity develops slowly because the host must generate a response to the introduced antigens. Its full effective response therefore takes time.
3. Memory B-cells and T-cells recognise the pathogen on later exposure. They support a faster, intensified response against the same pathogen.
4. Passive immunisation supplies preformed antibodies directly for protection. It differs from vaccination because those protective antibodies were produced outside the recipient.

Q7. Explain contact inhibition, its relationship to cancerous growth, and metastasis. [3 marks]

1. Contact inhibition is the restraint on uncontrolled growth produced when normal cells come into contact with neighbouring cells.
2. Cancer cells appear to have lost this restraint. Their continued division produces a mass of cells, called a tumour, rather than appropriately regulated growth.
3. Metastasis occurs when cells from a malignant tumour travel to distant sites, such as through blood, and establish new tumours there.

Q8. Define addiction and dependence, explain tolerance, and give one reason why withdrawal may require medical supervision. [4 marks]

1. Addiction is psychological attachment to effects of drugs or alcohol, such as euphoria or temporary well-being, which encourages repeated use.
2. Dependence is the tendency to develop an unpleasant withdrawal syndrome when a regular dose of the substance is abruptly discontinued.
3. Tolerance increases with repeated use, so higher doses are needed to produce the response. This can encourage increased consumption.
4. In some cases, withdrawal symptoms can become severe and even life-threatening. This possibility is a reason for medical supervision.

Key takeaways

• Health includes physical, mental and social well-being; prevention combines personal hygiene, safe food and water, vaccination and public sanitation.
• Match each disease with its pathogen and route of transmission; a vector and the pathogen it carries are different organisms.
• Plasmodium alternates between humans and mosquitoes, with sporozoites entering humans and gametocytes entering the mosquito during a blood meal.
• Innate immunity supplies non-specific barriers; acquired immunity recognises particular pathogens and produces a stronger response after repeated exposure.
• B-lymphocytes produce antibodies, while T-lymphocytes mediate cellular responses; active and passive immunity differ in the source of protective antibodies.
• HIV progressively reduces helper T-cell numbers, weakening resistance to infections; ordinary touch does not transmit the virus.
• Malignant tumours invade tissues and can metastasise; early detection allows successful cancer treatment in many cases.
• Drug, alcohol and tobacco abuse harm health; counselling, supportive relationships and professional care are important for prevention and recovery.

Test yourself

Which structures fill with fluid in bacterial pneumonia?

The alveoli, or air sacs of the lungs, fill with fluid, making respiration difficult.

Why is a housefly called a mechanical carrier in amoebiasis?

It carries the parasite from infected faeces to food, contaminating food that another person may consume.

Which cells secrete interferons, and which cells are protected?

Virus-infected cells secrete interferons, which protect non-infected cells against further viral infection.

What does H₂L₂ indicate in an antibody?

H denotes a heavy chain and L a light chain. The two subscripts indicate two chains of each kind.

Name the primary lymphoid organs and explain their role.

Bone marrow and thymus are primary lymphoid organs, supporting development and maturation of lymphocytes into antigen-sensitive cells.

Which antibody class is abundant in colostrum?

IgA is abundant in colostrum and provides passive immune protection to the newborn infant.

How does an allergen differ from a carcinogen?

An allergen triggers an exaggerated immune response; a carcinogen is an agent capable of inducing cancerous changes.

How does nicotine affect heart rate and blood pressure?

Nicotine stimulates adrenal release of adrenaline and noradrenaline, which increase heart rate and raise blood pressure.