Class 9 Science

Why Do We Fall Ill?


It is a state of being well enough to properly function physically, mentally and socially. In other words, health is a state of physical, mental and social well being.

If a person is physically healthy but is mentally unfit, he is not a healthy person. If a person is physically and metnally healthy but indulges in criminal activities, he cannot be terms a healthy person.


See Answer

1: (a) Flu, 2: (c) Arthritis, 3: (b) Antiviral, 4: (d) Antibiotic, 5: (a) Tuberculosis, 6: (b) Diarrhea, 7: (d) Ringworm, 8: (c) Body fluid, 9: (b) Liver, 10: (c) Lungs

Community Issues Affecting Our Health

Public Cleanliness

Public cleanliness is important for our health. If someone is living in a filthy neighborhood, he has a greater risk of being affected by some or the other epidemic.

Economic Condition

Economic condition of the country and society is important for our health. A sound economy ensures that a person gets a job befitting his/her skills. This ensures adequate earning to make his life comfortable. Lack of earning can have negative effect on health.

Social Equality and Harmony

If a person lives in a climate of mutual distrust among the members of society, it can have negative effect on health. On the other hand, living in a harmonious society has positive effect on someone’s health.


The term disease literally means disturbed ease. A condition in which the affected person is unable to carry out normal activities is termed as disease. This condition can depend on a person’s age and many other factors. For example, if an old woman is just able to walk comfortably in the market or garden, she can be termed a healthy person. On the other hand, if a child stops running around and jumping at the smallest stimulus, the child may not be healthy.

Symptoms: Apparent signs which give a clue about an underlying disease are called symptoms. For example; headache, fever, abdominal cramps, etc.

Types of Disease

Acute Disease

A disease which appears for a short duration is called acute disease. In most of the cases, an acute disease does not result in debility. Debility means physical weakness. Sometimes, debility can hamper normal physical activities of a person in significant way. Example: cholera, diarrhea, flu, common cold, etc.

Chronic Disease

A disease which lasts for many years is called chronic disease. Some of the chronic diseases may last for the entire life. A chronic disease often results in debility. Example: arthritis, tuberculosis, diabetes, hypertension, etc.

Infectious Disease

When a disease happens because of a microorganism, it is called infectious disease. Bacteria, protozoa, virus and fungi are the causes of diseases in this case. Example: diarrhea, tuberculosis, dengue, malaria, hepatitis, etc.

Non-infectious Disease

When a disease happens because of some malfunctioning in any organ and it is not because of any microorganism, it is called non-infectious disease. A non-infectious disease may happen because of a wrong lifestyle. Some non-infectious diseases may happen because of genetic factor, i.e. they are due to some faulty gene. Example: diabetes, hypertension, heart disease, arthritis, hemophilia, etc.

Infectious Diseases
BacteriaDiarrhea, tuberculosis, tetanus, whooping cough
VirusCommon cold, jaundice, hepatitis, flu, dengue
FungiRing worm, dandruff
ProtozoaMalaria, amoebic dysentery

Types of Antimicrobials


A substance which stops the growth of bacteria or kills the bacteria is called antibiotic. Antibiotic is given to treat or prevent bacterial infection. Some antibiotics damage the cell wall of bacteria and thus kill the bacteria. Example: streptomycin, tetracycline, amoxicillin, etc.


A substance which stops the growth of virus is called antiviral. Viral diseases are very difficult to treat. Very few effective antiviral medicines are available in market.


A substance which stops the growth of fungi or kills the fungi is called antifungal.

Means of Spreading the Infection:

By Air

Some of the pathogens (harmful and disease causing microbes) spread through air. For example; when a person suffering from common cold sneezes or coughs, viruses come out along with fine droplets of mucus. The virus then easily enters another person’s body. Thus common cold is spread from one person to another. Flu also spreads through air.

By Water

Some of the pathogens spread through contaminated food and water. Diarrhea and jaundice spread from one person to another through contaminated food and water.

By Sharing of Clothes

Ringworm spreads by sharing towels or clothes of the infected person.

Through Blood or Body Fluid

Some diseases spread from one person to another through blood or body fluid. AIDS and Hepatitis B come under this category. These diseases can spread if infected blood is used in blood transfusion. They can spread from an infected mother to the unborn child. They can spread through unprotected sex.

By Vectors

Some animals work as vehicles to transfer infection from one person to another. Such animals are called vectors. Malaria spreads in this way and mosquito plays the role of vector. The female anopheles mosquito spreads the malaria parasite from an infected person to a healthy person. Dengue is also spread by mosquito. When a mosquito bites an infected person, parasite enters the mosquito’s body along with blood. When the same mosquito bites a healthy person, parasite enters the body of the healthy person. Dogs are the vectors for rabies virus.

Affected Organs

Mode of infection often determines the infected organs. When a disease is spread through air then it is the respiratory system which gets infected. When a disease is spread through contaminated food or water then the digestive system gets infected. Jaundice and hepatitis B affect the liver. Tuberculosis usually affects the lungs. But some diseases end up affecting the whole body.