Why Do We Fall Ill Class 9 CBSE Notes - Chapter 13



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 provides protection against infection from pathogens, invading foreign substances and other toxins.

To know more about the Immune System, visit here.

Skin and Mucous Membranes

Skin and Mucous membranes act as the layer of defence. 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 kill the pathogens and protect our body free from pathogens and infections. There are of 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 the 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.

For More Information On How Does Our Immune System Work, Watch The Below Video:


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

To know more about Diseases, visit here.

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 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 objective.

Causes of Disease

Different types of Causes of Disease

Diseases can be caused due to pathogens such as virus or bacteria. Some diseases can also be due to internal factors such as genetic mutation.

Infectious Diseases

Diseases that are caused by pathogens and can spread to other individuals in the populations are called infectious diseases.

Non-infectious diseases

Diseases that cannot spread from one individual to another are called non-infectious diseases. Usually, these diseases are not caused by a pathogen.

To know more about Infectious and Non-Infectious, visit here.


Pathogens are external agents that cause diseases in other organisms. This pathogen includes harmful microbes or microorganism such as bacteria, virus, fungi or protozoa.

To know more about Pathogens, visit here.


Vectors are those organisms that carry a pathogen from the host to a recipient. Mosquito, rats and mice are some of the common vectors that carry infectious diseases.


Bacteria are microorganisms that are seen in almost all environmental condition. Not all bacteria are harmful to pathogens. Some bacteria are also beneficial to human beings. Bacteria are beneficial for, digestion, extracting antibiotics from them, nitrogen fixation, etc.

A few of the common diseases caused by bacteria are cholera, typhoid, pneumonia, tuberculosis, etc.

For More Information On Bacteria and Vectors, Watch The Below Video:


A virus is a microorganism that is always pathogenic in nature. They do not have molecular machinery to replicate without a host. Therefore, they enter the host cell and replicate and in the process destroy the host cell. A few of the common diseases spread by the viruses are cold, influenza, dengue fever AIDS, etc.


Fungi are a group of organisms which are eukaryotic in nature and saprophytic in nutrition. They could be either unicellular or multicellular organism. Many common skin infections such as ringworm, nail infection, etc are examples of Fungal diseases


A parasite is an organism that lives in another organism, called the host, and often harms it. It is dependent on its host for survival – it has to be in the host to live, grow and multiply.



AIDS stands for Acquired Immunodeficiency Syndrome. It is caused by the Human Immunodeficiency Virus. AIDS systematically destroys the immune system of the patient, leaving them vulnerable to the easiest of the diseases.

Prevention of Disease


Antibiotics are antimicrobial drugs produced from other organisms, such as fungus and some bacteria, which are used for treating against the harmful infections caused by pathogens or harmful microorganisms. These antibiotics functions by:

  1. Alteration of Cell Membranes.
  2. Inhibition Antimetabolite Activity.
  3. Inhibition of Nucleic Acid Synthesis.
  4. Inhibiting of Cell Wall Synthesis (a most common mechanism).
  5. Inhibition of Protein Synthesis (Translation) (second largest class).

Preventive Measures

The preventive measures can be taken to avoid infection of various diseases. The most common measure is the maintenance of hygienic condition.

To know more about Prevention of Disease, visit here.


Immunization is the process whereby a person is made immune or resistant to an infectious disease. Vaccines are the common means to immunize people.

To know more about Immunization, visit here.

Frequently asked Questions on CBSE Class 9 Science Notes Chapter 13: Why do we Fall Ill

What are the function of the ‘Immune System’?

1. Fight germs/diseases which are caused by the pathogen 2. Recognise/neutralise the harmful substances 3. Protect us against harmful environmental substances

What is a ‘macrophage’?

Macrophages are specialised cells involved in the detection, phagocytosis and destruction of bacteria and other harmful organisms.

What is a ‘Bacteriophage’?

A bacteriophage is a type of virus that infects bacteria. It is also known as the ‘bacteria eater’.

Leave a Comment

Your Mobile number and Email id will not be published. Required fields are marked *




Free Class