CBSE Science Revision Notes for Chapter 13 Why Do We Fall Ill


Class 9 Science Revision Notes for Chapter 13 Why Do We Fall Ill are created by experts at askIITians to help you understand the causes and cure of various diseases. This chapter helps you understand how important it is for you to take care of your health. It is an interesting chapter with topics like the difference between being healthy and being disease-free, acute and chronic diseases, causes of diseases, infectious and non-infectious diseases, means of spread of diseases, principles of prevention, principles of treatment, and immunisation. 


The revision notes for CBSE Class 9 Science Chapter 13 Why Do We Fall Ill are available for free on our website. These notes will be extremely helpful for every student because they include short, concise yet clear explanations for all the topics of the chapter. The notes are based on the latest syllabus and hence will help you prepare well for the exams. You can find pointwise explanations for every concept of the chapter which makes our notes easier to read. You can use these revision notes to revise the whole chapter before exams in just 20 minutes or less! 


At askIITians, we provide a plethora of study resources to the students for Class 9 Science. These include mind maps, flashcards, NCERT Solutions, NCERT Exemplar solutions, important questions, extra questions, and more. We also provide class 6, 7, 8, 9, 10, 11, 12 notes for Science and Maths to the students. 


Online Revision Notes for Class 9 Chapter 13 Why do we Fall Ill


The activities performed inside our body by the internal organs are all interconnected. For instance, the heart always forms the blood in the body, the brain always thinks, the kidney filters out waste from our body and so on. If any of these activities stop, this would affect the whole functionality of our body.


What is health?

It is a state of being well so that one can perform physical, social and mental functions properly. For Example, we say a person is healthy if they can perform their day to day tasks very well.

Figure 1: What is health?


Is health dependent on personal and social issues?

Yes, we cannot achieve health solely on our own. Every organism in this world depends upon another or the environment for its better health.


Factors that determine better health:

  • We always stress the fact that the environment surrounding us should be healthy otherwise it may lead to harmful diseases. Therefore, we can say that public cleanliness is an important factor for the better health of people in society.
  • Another important factor for proper health is food. Now, we can have food only if we have the money to buy it and for that, we need to work. Therefore we can say that good economic conditions of society and employment are needed for better health.
  • Lastly, we can stay healthy if we are living a tension free life. How can we expect a healthy environment around us if everyone keeps on ill-treating each other? Therefore, we can say that a good social environment is required for better health.


What do you mean by a disease?

  • We can say that a disease is any abnormality or disturbance caused in our body.
  • A disease is not caused by any external injury but can be caused by an external factor like germs.
  • Sometimes internal dysfunctionality of our body may also lead to diseases.
  • A disease generally has some Symptoms and Signs associated with it. For Example, Pain, Swelling and Fever are some common symptoms.


How is being disease-free different from being healthy?

Figure 2: Healthy and Disease-Free


How can we stay disease-free?

We can stay disease-free by maintaining good health that is,

  • by having proper food or a balanced diet
  • by keeping the environment clean
  • maintaining personal hygiene


How can we identify a disease?

  • Disease is associated with symptoms. In other words, our body shows certain indications with which we can assume that we may be suffering from a disease.
  • We know that different parts of our body perform different functions.
  • Any of these functions are disturbed, we can say that something is wrong within our body or something has changed in it. This is a symptom of getting a disease.
  • Symptoms just indicate that there is a disease. They do not indicate the exact type of disease.
  • The doctors often look for the signs of a disease in order to find out the exact problem. These signs, unlike the symptoms, are more definite indications of a disease. Sometimes laboratory tests are also done in order to find a disease.


Types of Diseases: Acute Diseases and Chronic Diseases


Acute Disease

Chronic Disease

Acute diseases last for only a short period of time.

Chronic diseases last for a long period of time

It is caused randomly.

It is caused in due course of time.

It does not cause a major effect on general health.

Example: Cough, dysentery.

It causes a major effect on general health.

Example: Elephantiasis, heart disease, tuberculosis.



How do chronic diseases affect our health?

  • Chronic diseases take relatively a long period of time hence they are likely to affect our general health as well.
  • They may hinder the growth of children or increase stress in adults.
  • They can make us feel tired all the time.
  • They can also lead to an increase or decrease in weight.
  • They can also affect our day to day activities and the ability to learn new things.
  • Hence, we can say that they have long-term effects on health than acute diseases.


What causes diseases?

We know that diseases can be caused by two factors:

Internal factors in our body such as

  • Hormonal imbalance
  • Allergic Reaction
  • Genetic disorder
  • Malfunctioning of body organs

External factors such as

  • Unhealthy diet
  • Disease-causing germs (bacteria, viruses, fungi)
  • Pollution in the environment
  • Unhealthy lifestyle

Based on the following there are two broad categories of causes of diseases –


1. Immediate Causes and Contributory Causes

Immediate Causes

Contributory Causes 

These are the actual causes that are responsible for a disease. These causes can be infectious as well as non-infectious. 

These are not the direct causes but factors that contribute to causing disease.

For Example, Bacteria, Fungi, Viruses, Germs etc. 

For Example Poor diet, Unhealthy lifestyle, Polluted environment etc. 



2. Congenital and Acquired Diseases 

Congenital diseases are present since birth. For eg hole in heart of an infant. Acquired Diseases occur after birth. Based on their ability to spread from one individual to another, Acquired diseases are of two types:


Infectious and Non-infectious causes of Disease 

Infectious Causes

Non-infectious Causes

These are the extrinsic or external disease-causing factors. 

These are the intrinsic or internal disease-causing factors. 

They lead to infectious diseases and can affect the whole community as they are transferable from one person to another.

They lead to non-infectious diseases and do not affect the whole community as they are non-transferable from one person to another. 


Difference between Infectious and Non-infectious Diseases:

Infectious or Communicable Diseases

Non-infectious or Non-Communicable Diseases

They are caused by an attack of pathogens.

They are not caused by pathogens.

The diseases are brought about by extrinsic or external factors.

The diseases are mostly bought by intrinsic or internal factors.

Infectious diseases can pass from a diseased person to a healthy person.

Non-infectious diseases cannot pass from one person to another.

Transmission of infection occurs through direct contact or some medium (air, water, vectors).

Transmission is absent, However, hereditary diseases are transmitted from parent to offspring.

Community hygiene can reduce the incidence of infections diseases

Community hygiene is ineffective in reducing the incidence of non-infectious diseases.

Example: Cholera, Tuberculosis (TB), Pneumonia, Chickenpox.

Example: High blood pressure, Heart, Disease, Cancer.


Infectious Agents

The pathogens or microbes that cause infectious diseases are also called Infectious Agents.

Figure 3: Infectious Agents


Why do we need to understand the characteristics of these infectious agents?

If we understand the characteristics we can treat the diseases caused by them in a better way. We can find out which medicine would work on which infectious agent. Some common characteristics of infectious agents are:

  • Viruses live inside the host body
  • Bacteria do not generally live inside the host body
  • Virus, bacteria and fungi multiply quickly
  • Worms do not multiply quickly


How can penicillin help fight against bacteria?

Antibiotics are generally used to block the growth of bacteria. The bacteria cells grow by creating a cell wall that protects them. Penicillin is an antibiotic that prevents the growth of the cell wall and hence bacteria die easily. Penicillin is used for fighting against different kinds of bacteria.


Several types of diseases

Based on the means of spread of a disease, we can classify it into different categories:

Type of Disease

Causing Factor


Airborne diseases

They are caused by germs, bacteria or virus in the air

Common cold, tuberculosis

Foodborne diseases

They are caused by germs (bacteria, toxins, viruses, fungi) present in the food 

Food poisoning, Typhoid

Waterborne diseases 

They are caused by drinking contaminated water 

Cholera, Amoebiasis 

Lifestyle diseases

They occur because of poor or unhealthy lifestyle

Heart disease, Diabetes 

Vector-borne Diseases

they are caused due to animals that carry infectious agents from a sick person to another person

Malaria, Dengue Fever

These animals that act as an intermediate between disease-causing germs and people are called vectors. 

Sexually transmitted diseases

They are caused due to sexual contact from one person to another

AIDS, Syphilis 



Where do the disease-causing germs live in our body?

  • There are several tissues and organs in our body where these microbes can get attached to.
  • Generally, the point of entry decides where they will go. For instance, any microbe that enters through the nose is likely to settle in the lungs.
  • This can also be seen through the signs and symptoms of a disease as only those organs and tissues issues seem affected where these microbes enter.
  • But there are some other common diseases also that are not tissue-specific.
  • Inflammation – Our body has an immune system that creates the cells that can fight against the disease-causing germs. This process of recruiting cells to kill the infectious agents present in our body is called Inflammation. The inflammation process shows different effects on our body such as fever or swelling.
  • Hence, we can say that the likelihood we are going to be affected by the disease is determined by the immune system of our body.


Principles of treatment

Figure 4: Principles of Treatment


We may treat infectious disease in two ways:

  • By killing the infectious agents
  • By reducing the effects of the disease or reducing the symptoms


We can reduce the symptoms in the following ways:

  • By taking rest to conserve our energy so that our immune system can fight against the disease
  • By taking medicine to reduce the common symptoms such as fever or pain and hence reducing the disease


We can kill the infectious agents in our body in the following way:

  • By taking medicines that can kill them such as antibiotics or antiviral medicines


Antibiotics aren’t effective against viruses?

Antibiotics commonly work by blocking the biochemical pathways that are important for bacteria. Thus these inhibit the growth of bacteria, hamper their metabolism and kill them. Antibiotics do not work on viruses because viruses do not use the biochemical path and use host cell machinery for making proteins.

However, the most effective way to treat viral infections and disease is vaccination as it can prevent a person from getting the disease in the first place.


The principles of prevention

Are there any limitations of treating infectious agents via medicines?

Yes, there are three limitations:

  • Our body functions might not be able to recover easily
  • This treatment takes time hence it can affect our daily activities
  • An infectious disease may transfer from a person who is suffering the disease to another in the meanwhile of the treatment,

Therefore, we should find out ways to prevent these diseases in the first place. There are two ways to prevent diseases:

  1. Preventing exposure to these infectious agents
  • Waterborne diseases can be prevented by always having safe and pure drinking water
  • Airborne diseases can be prevented by avoiding overcrowded places in keeping the environment clean
  • Vector-borne diseases can be prevented by keeping our surroundings clean and maintaining public hygiene
  1. Strengthening the immune system so that if any infectious agents enter our body can fight back. This can be made possible by having healthy food.


What is the Immune System?

  • The immune system is a network of cells, tissues and organs that work together to protect our body from diseases. We may consider the immune system as a defence system of our body.
  • The immune system looks out and destroys the disease-causing germs in our body with the help of special cells called white blood cells. These cells are present in the blood and hence circulate throughout the body and monitor it.
  • The germs or any foreign substance that enters our body are called Antigens. As the immune system recognizes these antigens, it releases antibodies that lock the antigens and then destroy them with the help of other cells.
  • The ability of a body to resist disease with the help of antibodies is called Immunity.


Discovery of Helicobacter Pylori as a Reason for Ulcer

A peptic ulcer is a sore that occurs on the inner lining of the stomach. In 2005, Barry J. Marshall and J. Robin Warren received a Nobel Prize for discovering that Helicobacter Pylori bacteria are a cause of such ulcers. Before that, it was believed that stress and lifestyle are major causes of such ulcers. The  Helicobacter Pylori bacteria weakens the mucous lining of the stomach which lets the acid present inside the stomach get through the inner sensitive lining. The acid and bacteria together irritate the lining and cause a sore or ulcer.


A Specific Method to Strengthen the Immune System

  • The above methods were just general ways of preventing any disease. A specific method to strengthen the immune system and hence prevent disease is vaccination.
  • Whenever our body is affected by a disease, our immune system not only fights against it but also remembers how to respond when the same disease-causing microbes affect our body the next time.
  • Similarly, vaccination contains an agent that is similar to the disease-causing agents (weak or killed microbes). As it enters our body, our immune system learns to fight against it and hence prevents us from actually getting infected when the actual disease-causing microbes enter our body.
  • Today, vaccines are available for enhancing our immune system against various diseases such as polio, chickenpox and measles. 

Diseases - Causes, Control and Prevention

Figure 5: List of Diseases


CBSE Class 9 Science Revision Notes for Why Do We Fall Ill FAQs


  1. Why study Chapter 13 Why Do We Fall Ill in Class 9 Science? 

Chapter 13 Why Do We Fall Ill helps you understand the meaning of living a healthy life or being healthy. It gives information about various diseases, their causes, prevention and cures. It also helps you understand why public hygiene is important and sheds light on immunisation. This chapter is important in daily life as well as in advanced studies. 


  1. How should I study CBSE Class 9 Science Chapter 13 Why Do We Fall Ill? 
  • The best method to learn the important topics of this chapter is to study our online revision notes for Why Do We Fall Ill. 
  • You must also read the NCERT textbook to understand what this chapter is about and what are the important terms used in it. 
  • Once you have understood the chapter, you should solve the NCERT questions and NCERT Exemplar problems to solidify your understanding. 


  1. I need more help in CBSE Class 9 Science. What should I do? 

You can join askIITians online coaching for Class 9 Science where we teach every concept in detail. There are live, interactive classes, doubt removal sessions, discussions and tests to help you master every concept of Class 9 Science. Along with this, we provide a plethora of study materials like revision notes, NCERT Solutions, NCERT Exemplar Solutions, important questions and more to help you prepare every topic for exams. 


  1. What are the features of Class 9 Science Revision Notes for Why Do We Fall Ill? 
  • The revision notes are created by Science experts at askIITians. 
  • These notes provide easy to understand explanations for every topic included in the chapter. 
  • The notes are prepared according to the latest CBSE syllabus and exam pattern. 
  • They include pointwise explanations for all the topics of the chapter. 

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