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Why do we fall ill? - Class 9 Science Notes

What is Health?

Health is a state of complete physical, mental and social well-being. It is not merely the absence of diseases. It is a state of complete physical, mental and social well being. Health of a person depends upon one’s personal habits as well as physical and social environment.

What are the basic conditions for good health?

  1. Proper balanced and nutritious diet.

  2. Personal hygiene.

  3. Clean environment and surroundings.

  4. Healthy air, no pollution in the surrounding.

  5. Regular exercise.

  6. Proper rest.

  7. Good standard of living and economic status.

What is a Disease?

When the body is not at ease i.e., comfortable then it is said to have a disease. A disease is a condition of the body or a part of it in which functions are disturbed or damaged. When there is a disease, the functioning or appearance of one or more systems of the body changes.

How are diseases diagnosed?

Diseases are diagnosed with the help of symptoms and signs.

What are Symptoms?

They are manifestations or evidence of the presence of diseases. They are in the form of structural and functional changes in the body or body parts. On the basis of symptoms, physicians search for definite signs of the disease. The cause of disease can be found by tests etc.

Depending on the duration—disease is classified as acute or chronic.

  • Acute disease: Diseases that last for only short period of time, e.g., headache, common cold etc. Being of short duration an acute disease is unable to cause major effect on health.

  • Chronic disease: Diseases that last for long time, are called chronic diseases, e.g., elephantiasis, tuberculosis, etc. They generally develops slowly with a milder course in the beginning. It builds up with time. Due to prolonged duration, it damages the affected organ. There is often a loss of weight accompanied by feeling of tiredness.

What are the causes of diseases?

Immediate cause: The organisms that enter our body and causes disease is called immediate cause. For example, virus, bacteria, protozoa etc.

Contributory cause: The secondary factors which led these organisms enter our body are called as contributory cause. For example, dirty water, unhealthy surroundings, contaminated food, improper nourishment, poverty, poor standard of living etc.

  • Congenital diseases are the diseases that are present since birth either due to defective development of embryo or defective inheritance.

  • Acquired diseases are those diseases which are picked up after birth. Depending upon their ability or inability to spread from one individual to another acquired diseases are of two kinds, infectious and non-infectious.

Diseases may be due to infectious and non-infectious causes.

(a) Infectious causes: Diseases where microbes are the immediate causes are called infections diseases. The infection spreads from one person to another. For example: malaria, cholera, food poisoning etc.

Disease causing organisms are called pathogens.

(b) Non-infectious causes: Some diseases that do not spread in the community, but remains internal are called non-infectious diseases. For example: cancer, genetic abnormalities, high blood pressure etc.

Infectious diseases (Communicable diseases): When a disease causing organism enters our body it causes infection, it multiplies and grows in the body of host.

Infectious diseases spread through:

(a) Air: Causes air-borne diseases due to bacteria, virus e.g., common cold, influenza, measles, tuberculosis.

(b) Food and water: Is caused due to contaminated food and water that contains bacteria, virus, worm etc. Example, cholera, typhoid and hepatitis.

(c) Contact: Many diseases spread by contact of infected person with the healthy person. Examples, fungal infection, scabies etc. AIDS and syphilis spread due to sexual contact.

(d) Body fluids: Body fluids like blood, semen, mother milk when infected can also cause disease. Example, AIDS.

The various causes of diseases are pathogens, lack of nutritious diet and lack of public health services.

The infectious agents are bacteria, viruses, fungi, protozoans and helminths. The infectious agents may attack the same organ where they have entered or they may enter some other organ. The disease causing microbes enter the body through different means i.e., through nose, mouth, genital openings or through blood stream (wounds).

In most cases the infected organ or tissue-specificity depends upon the portal of entry. Disease will spread in the body only when the microbe enters the body in sufficient number.

Some of the common effects of infectious diseases are fever, inflammation, diarrhoea etc.

Antibiotics: These are the chemicals (medicine, drugs) that block biochemical pathways important for bacteria. They are used for diseases caused by bacteria.

Inflammation: When an active immune system releases many cells to the affected tissue to kill-off the disease-causing microbes it is called inflammation. Local effects caused on body due to inflammation are—swelling, pain, fever and redness.

Principles of treatment

(1) To reduce the effects of the diseases.

(2) To kill the cause of the disease i.e., to kill the microbes like bacteria, fungi, protozoa. We kill microbes by using antibiotics. Antibiotics are certain chemicals obtained from a group of microorganisms that kill or retard the growth of another microbe.

Antibiotics such as penicillin blocks the process of cell wall formation in growing bacteria as a result of which they cannot multiply and ultimately die.

Viruses do not form a cell wall or use such pathways and hence antibiotics do not work against viruses. Despite this limitation there are now effective antiviral drugs available in the market; for example, drugs that keep HIV infection under control.

Principles of Prevention:

(1) General method (2) Specific method

> General ways of preventing infections relate to preventing exposure.

Prevention of exposure can be done in following ways:

  1. For air-borne infections—avoid visiting public place, cover your nose and mouth while coughing.

  2. For water-borne infections: Drink, clean and boiled drinking water.

  3. For vector-borne infections: Keep the surroundings clean, do not keep any puddle of water open in the surrounding as it allows the breeding of mosquitoes.

  4. Self immune system that can fight off and kill microbes when it enters our body.

  5. Availability of proper and sufficient food for everyone.

> Specific ways: By giving vaccines, childhood immunization that is given to the children for preventing infectious diseases.

Vaccines are heat killed or chemically weakened germs which when injected in our body develops memory for a particular infection in our body It stimulates the formation of antibodies by the immune system which prevents any subsequent exposure to the infecting microbe from turning into an actual disease.

Vaccines are available against number of diseases such as tetanus, diphtheria, whooping cough, measles, polio, hepatitis and so on.

Detailed notes on Types of Diseases

The word disease (dis-ease) literally means disturbed ease or uncomfortable. Thus disease can be defined as "Malfunctioning of the body or a part of it due to one reason or the other".


Disease is a condition of the body or a part of it in which functions are disturbed or deranged.

Signs and symptoms of a disease

Symptoms of disease

Signs of disease

A symptom is a change in normal functioning of the body which is felt by patient but can not be observed by the doctor, a patient has to tell the doctor about these e.g. stomachache, headache

A sign of a disease is a change in the bodily function or structure that can be observed. e.g. Rashes, swelling of gland

Signs and symptoms help in the diagnosis of a disease Diagnosis means the process of identifying a disease. In many case doctor also needs the help of laboratory tests of blood, urine, stool etc. and diagnostic images like x rays to diagnose a disease.

Pathogens - Disease causing organisms are called pathogens or infectious agents.

Vectors - Animals or insects that carry the infectious agents from one person to another and spread a disease are called vectors.

Insect vector

Disease Transmitted

1. Mosquitoes (i) Anopheles (ii) Culex (iii) Aedes

(i) Malaria

(ii) Filariasis

(iii) Yellow fever, Dengue, Filariasis

2. Flies (i) House fly (ii) Sand fly (iii) Tse-tse fly

(i) Thyphoid, Diarrhoea, Dysentry, Cholera, Tuberculosis, Conjunctivitis

(ii) Kala azar, Oriental Sore

(ii) Sleeping sickness

3. Louse

Epidemic typhus

4. Rat flea

Bubonic plague

Do you know?

Hippocrates :

First to look for scientific explanation for diseases. He is remembered today as FATHER OF MEDICINE.

Susruta :

He used non poisonous leeches as an anticoagulant during surgery so he is called father of surgery.

Charaka :

He first gave concept of digestive metabolism and immunity. He is known as Father of Ayurveda [Ayu- Life, veda- knowledge]

Rudolf virchow :

Father of Modern pathology.

Father of Immunity :

Edward Jenner (small pox vaccine)

Father of Modern bacteriology:

Robert Koch (Anthrax T.B., cholera)


A disease that occurs suddenly and lasts for a short period of time is called an acute disease e.g. common cold Malaria disease.

A disease that lasts for a long time is called chronic disease e.g. tuberculosis.

Disease and its cause

Human health is affected by various factors causes or sources, these factors can be of two major types :-

(i) Intrinsic or internal factors : Factors inside the body or within the body.

(ii) External or extrinsic factors : Factors out side the body.

Classification of some common diseases of human

Sr No.

Type of Disease

Causes of Disease




Temporary or permanent damage to body parts.

Bone fracture, leprosy



Invasion of the body by other organisms (viruses, bacteria, fungi protozoans, nemetodes, insects, etc.)

Typhoid, diarrhoea, malaria, hepatitis, rabies, AIDS, T.B., polio, influenza, cholera.



Inadequate diet

Kwashiorkor, Marasmus, scurvy, rickets, anaemia, goitre, xerophthalmia, beriberi.



Defective genes passed on from parents to offspring

Haemophilia, sickle-cell anaemia, cystic fibrosis.



Organs and tissues wear away and do not work so well with age.

Arthritis, poor sight and hearing defects.



Brain damage or psychological.

Depression, paranoia.



Social interactions with family friends and strangers.

Drug dependence alcoholism, smoking, unhealthy life, social isolation.

Means of spread :- Disease causing microbes can spread through the air. This occurs through the little droplets thrown out by an infected person who cough or sneezes. Disease can also be spread through water. When infectious agents get mixed with drinking water.

Some diseases are spread by sexual contacts or blood transfusions animals can also carry infectious agents.

Organ specific and Tissue specific Manifestations

Different species of microbes enters through various points of body and there are many possible places organs or tissues where they could go.

If they enter from the air via nose, they are likely to go to the lungs. If they enter through the mouth they can stay in gut lining.

Malaria causing microbes enter through a mosquito bite and will go to the liver and then to Red blood cells but virus of Japanese encephalitis or brain fever will similarly enter through mosquito bite will enter in brain or goes to infect brain.

The signs and symptoms of a disease will thus depend on the tissue or organs which the microbes target.

Bacterial disease


Food poisoning by bacteria Salmonella is called salmonellosis. Bacteria Salmonella is present in the sick farm animals. So the infection of bacteria Salmonella which causes food poisoning is contracted from the sick farm animals through their contaminated food products such as milk, eggs and meat.

Toxins released by Escherichia coli cause mild diarrhoea to severe dehydration. Shigellosis, caused by Shigella, is characterized by frequent passage of stools with blood and mucus and abdominal cramps.

Clostridium botulinum causes food poisoning or botulism from taking preserved foods (dibba-bund food).



CANCER (crab = cancer)

Uncontrolled , abnormal and excessive mitotic division of cells is called cancer.

Oncology: Study of cancer is called oncology.

Cancerous cells: The abnormal and undifferentiated cells are called cancerous cells. Cancer is a non infections disease but cancer cells spread from one part to another part of body through blood lymph etc.

Tumour or Neoplasm: The abnormal tissue that grows continuously forms a tumour. Tumour's are of two types (1) Benign, (2) Malignant

Carcinogens: The factor that induce cancer are called carcinogens they include

(i) Pollutants

(ii) Radiations

(iii) Tobacco

(iv) Biological agents

(v) Mechanical agents.

Symptoms: Initially a lump or swelling or sore development followed by unexplained loss of weight, difficulty in swallowing, Persistent cough etc. Types of cancer depending on the organs affected

(i) Carcinoma : In skin glands, lungs, breast, pancreas and stomach

(ii) Sarcoma : In connective tissues, bones, muscles.

(iii) Leukemia : Increase in no. of white blood cells.

Metastasis: The stage of cancer in which it becomes malignant and spreads to different body is called metastasis.

Diagnosis can be done by endoscopy and biopsy

Cure : (i) Radiation therapy (ii) Chemo therapy (iii) Surgery


AIDS: Acquired immuno deficiency syndrome.

Casual Agent : Human immuno deficiency virus H.I.V. This virus belong to retrovirus group of viruses.

HIV gradually reduces the efficiency of human immune system. This make the body vulnerable to other life threatening disease that finally causes patient's death.

How does a person get infected with HIV ?

This virus is transmitted through blood, semen and breast milk.

Thus a person can get HIV by:

(i) Transfusion of infected blood.

(ii) Sharing of needle with an infected person.

(iii) Having sexual intercourse with an infec