Why Do We Fall ill Class 9 Notes PDF

Why Do We Fall ill Class 9 Notes PDF Download

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Why Do We Fall ill Class 9 Notes PDF Summary

Greetings to all, today we are going to upload the Why Do We Fall ill Class 9 Notes PDF for the assistance of the students as well as teachers. In this editorial 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 crisp and brief explanations of all these topics to assist you to learn easily and in an organized manner. At the end of the notes, you can try the questions asked from the discussed set of topics and assess your preparedness.

Table of Content – Why Do We Fall ill Notes PDF

Sr. No. Topics
1. Introduction

  • Health
  • The Immune System
  • Skin and Mucous Membrane
2. WBC

  • Macrophages and NK Cells
  • Dendritic Cells
  • Inflammatory Response
3. Diseases

  • Acute Diseases
  • Chronic Diseases
  • Symptoms and signs of Diseases
4. Causes of Disease

  • Different types of Causes of Disease
  • Infectious Diseases
  • Non-infectious diseases
  • Pathogens
  • Vector
  • Bacteria
5.  Virus

  • Fungi
  • Parasites
6. Infections

  • AIDS
7. Prevention of Disease

  • Antibiotics
  • Preventive Measures
  • Immunization

Why Do We Fall ill Class 9 Notes PDF – Full Explanation



Health is a complete state of physical, mental, and social well-being.

The Immune System

An immune system is the part of the body that protects against infection from pathogens, invading foreign substances, and other toxins.

 Skin and Mucous Membranes

Skin and Mucous membranes act as the layer of defense. While skin protects the body externally, mucous membrane protects the insides of the body.


WBC- White blood cells are called Leucocytes or Leukocytes. They are the important components of our immune system and are present in the blood and lymph.  They function by attacking and killing pathogens and protecting our body free from pathogens and infections. There are different types of and are classified based on the location as well. The different types of White blood cells found in the blood are neutrophils, lymphocytes, monocytes, basophils, ad eosinophils. These blood cells have specialized functions.

Macrophages and NK Cells

Macrophages are large and specialized cells of the immune system.  These cells are produced in response to infections or due to the development of damaged or dead cells. They attack cancer cells through destruction and ingestion. Natural Killer Cells bind to the enemy cell and they dissolve the membrane so the cell can’t function.

Dendritic Cells

Dendritic cells create memory and carry information about the pathogens to the liver, spleen, lymph nodes.

Inflammatory Response

An inflammatory response is seen in the body when it gets injured or infected by a pathogen. Inflammations help in the localization of the issue and prevent it from spreading.


A disease is discomfort the body experiences physically or mentally. A disease could be caused due to external or internal factors. The two types of disease are:

Acute Diseases

Acute diseases are those which last for a very short time. These diseases can be fatal and are usually caused by an external agent.

Chronic Diseases

Chronic diseases are those which last for a long time. They take a lot of time to heal and can be caused by any external or internal factor.

Symptoms and signs of Diseases

When the body gets diseased, it shows certain symptoms and signs of illness. These symptoms and signs help in the identification and diagnosis of the disease. A symptom is felt by the affected person while signs can be detected by another person. A symptom is thus, subjective and a sign is an object.

Read more – Why Do We Fall ill Notes PDF

Why Do We Fall ill Class 9 Notes PDF Questions  And Answers

Question 1. State any two conditions essential for being free of disease.
Answer. The two conditions essential for being free of disease are:

  1. Personal and -domestic .hygiene.
  2. Clean environment and surroundings

Question 2. Are the answers to the above questions necessarily the same or different? Why?
Answer: The answer to the above questions are different because a person may be free of disease but his mental, social or economical health may not be good.

Question 3. List any three reasons why you would think that you are sick and ought to see a doctor. If only one of these symptoms were present, would you still go to the doctor? Why or why not?
Answer: The 3 reasons why one would think that he is sick are—(1) headache, (2) cold and cough, (3) loose motions.
This indicates that there may be a disease but does not indicate what the disease is. So one would still visit the doctor for the treatment and to know the cause of the above symptom.
Even in the case of a single symptom, one needs to go to the doctor to get proper treatment.

Question 4. In which of the following case do you think the long-term effects on your health are likely to be most unpleasant?

  • If you get jaundice
  • If you get lice
  • If you get acne.

Answer: In the above cases, lice and acne are acute problems of our health that can be cured in a short duration. But jaundice is the disease that can have the most unpleasant effect on our health as it affects the most important organ of our body i.e., the liver. This disease is a chronic one.

Why Do We Fall ill Class 9 Notes PDF Extra Questions?

 Question 1.
Give one local and one general effect of the inflammation process.
Swelling or pain is a local effect. Fever or headache is a general effect.

Question 2.
Name the organism causing the following diseases:
(a) Kala-azar
(b) Sleeping sickness
Kala-azar is caused by Leishmania; Sleeping sickness is caused by Trypanosoma.

Question 3.
What are congenital diseases?
The diseases which are present in a person since birth are called congenital diseases.
Example: color blindness.

Question 4.
It was diagnosed that a patient has lost the power of fighting any infection.
(i) Name the disease the patient is suffering from.
(ii) Name the pathogen responsible for the disease.
(i) AIDS.
(iii) HIV is the causative organism of AIDS. [HIV-Human immunodeficiency virus]

Question 5.
What is immunity?
The ability of the body of a person to fight against the disease-causing organisms is called immunity. Name any disease that can be prevented by using vaccines.

Question 6.
The diseases can be prevented by using vaccines.
The diseases which can be prevented by using vaccines are polio, smallpox, diphtheria, tetanus, measles, etc.

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