CBSE Class 9 Science, Why Do We Fall Ill: Chapter notes

Get here the chapter notes on CBSE Class 9 Science: Why Do We Fall Ill. These notes are helpful to revise the huge syllabus in a short time and make an effective preparation for the periodic tests and annual exams.

Created On: May 26, 2020 09:49 IST
Class 9 Why Do We Fall Ill: Chapter Notes
Class 9 Why Do We Fall Ill: Chapter Notes

In this article you will get the CBSE Class 9 Science, Why Do We Fall Ill: Chapter notes. These chapter notes are prepared by the subject experts and cover every important topic from the chapter. You will get a crisp and brief explanation of all these topics so as to help you learn easily and in an organised manner. At the end of the notes you can try the questions asked from the discussed set of topics and assess your preparedness.

Main topics covered in this part of CBSE Class 9 Science, Why Do We Fall Ill: Chapter Notes, are:

  • Health
  • Disease and its Types
  • Causes of disease
  • Antibiotics
  • Principle of Treatment
  • Principles of Prevention

Also Read: CBSE Class 9 Complete Study Material for 2020-2021

Keynotes for Chapter - Why Do We Fall Ill, are:


Health is a state of complete physical, mental and social wellbeing.


Any disturbance in the structure or function of any organ or part of body.

It may be caused due to the attack of pathogens (virus, bacteria), lack of nutritious diet/balanced diet and lack of public health services.

CBSE Class 9 Science Syllabus 2020-2021

Types of Disease

Diseases are basically of two types. They are:

(i) Acute Disease

The disease which lasts for only a short period of time is called Acute Disease.

For example: Influenza (Flu), Common cold, etc.

(ii) Chronic Disease: The disease which lasts for a long period of time is called Chronic Disease

For example: TB, Cancer, etc.

Difference between Acute Disease & Chronic Disease

S. No.

Acute Disease

Chronic Disease



They are short duration diseases.

They are long lasting diseases.


Patient recovers completely after the cure.

Patient does not recover completely.


There is no loss of weight or feeling of tiredness afterward.

There is often loss of weight of feeling of tiredness.


There is short-duration loss of work and efficiency.

There is a prolonged loss of work and efficiency.


Causes of Diseases: 

Causes of diseases are explained below:

Immediate and Contributory Cause

  • Immediate cause: The organisms that enter our body and cause the disease are termed as an immediate cause. For example, virus, bacteria, protozoa etc.
  • Contributory cause: The secondary factors which led these organisms enter our body are termed as a contributory cause. For example, dirty water, contaminated food, improper nourishment, poor standard of living, etc.

Infectious and Non-infectious Cause

Infectious causes: They include the agents like microbes or micro-organisms as the immediate causes which spread the disease from an infected person to a healthy person.

Non-infectious causes: The disease which does not spread by contact between infected and healthy person through air and water, is called non-infectious disease.

Infectious and Non-infectious Diseases

Infectious Diseases: The diseases which spread due to infection by micro-organisms are called infectious diseases.

When a disease causing organism enters our body it causes infection, it multiplies and grows in the body called host and micro-organisms multiplies in the host body.

For example: Tuberculosis, tetanus, Common cold, AIDS, etc.

Non-infectious Diseases: The disease which does not spread by contact between infected and healthy person through air and water, is called non-infectious disease.

For example: Cancer, genetic abnormalities, high blood pressure, etc.

Means of Spreading Infectious Diseases:

S. No.

Type of Disease



Air born Diseases

Common cold, influenza, measles, tuberculosis.


Water born diseases

Cholera, typhoid, hepatitis.


Sexual Diseases

AIDS, Syphilis.


Animal born Disease



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

Antibiotics do not work against viral infections as the viruses do not have their own biochemical mechanisms but depend upon that of the host.

Principles of treatment
1. To reduce the effects of the diseases

By taking medicines to bring down the effects of the disease like fever, pain, etc. and by taking complete rest to conserve the body’s energy.

2. To kill the cause of the disease

By taking suitable antibiotics and drugs which kill the disease causing microbes.

Principles of Prevention
1. General method:

They involve (a) Preventing exposure against microbes (b) By boosting the immune system

(a) Preventing exposure against microbes can be done in the following ways:

For air-born microbes we can prevent exposure by avoiding overcrowded places. For water-borne microbes we can prevent exposure by not using contaminated water.

For vector-borne infections we can prevent exposure by providing a clean environment.

(b) By boosting the immune system:

Our immune system plays an important role in fighting against the microbes that enter our body. strengthening the immune system can help in preventing many diseases. 

So, proper nourishment or healthy diet that includes all the necessary nutrients as well as vitamins and minerals is necessary for better functioning of our immune system.

2. Specific method
It involves the prevention method directed against a particular disease.

It is done by immunisation which is the process of introducing a weakened pathogen inside the body of the host to make his/her immune system to produce antibodies against that particular disease so that the next time even if the disease will strike the host’s body with full vigor, the body will be able to protect itself with the help of these antibodies.

Try the following questions:

Q1. What is called vector. Give one example.

Q2. What is Vaccination? Give the details, how it works in human body. 

Q3. State any two conditions essential for good health.

Q4. Differentiate between communicable and non-communicable diseases. Give one example of each.

Q5. Name the pathogens which can cause acne and sleeping sickness.

Q7. What are the different means by which infectious diseases are spread?

Q8. What do you mean by active and passive immunisation? 

Q9. State two principles of treatment of a disease.

Q10. Why making anti-viral medicines is harder than making anti-bacterial medicines?

Also Read: CBSE Class 9 Science Chapter Notes - All Chapters

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