NCERT Class 9 Science Chapter 13 Why do we fall ill Notes, Explanation, Question Answers

why do we fall ill


CBSE Class 9 Science Chapter 13 Why do we fall ill Notes, Explanation with Video, Question Answers


Class 9 Science

Chapter 13 – Why Do We Fall ill

Topics to be covered



Why Do We Fall ill Video Explanation



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We all do a lot of activities like exercises,eating meals, fruits, etc. in order to achieve good health. But do we know exactly what is good health? Are we actually healthy? Let us learn about the meaning of being healthy and unhealthy. We know that our body is formed of many organs and also, there are so many systems operating in the body. If one organ stops working, then other functions are also affected. This leads to uneasiness.


It is a state of physical, mental and social well being. For being healthy, one needs to eat a balanced diet, take rest, take proper sleep, live in a hygienic place, etc . But sometimes, certain signs appear in a person that cause uneasiness. This uneasiness is called disease.


It is a state when the body is not functioning properly due to one or more reasons. We do not actually feel comfortable in carrying out routine tasks like there can be pain in the body or body part or we may suffer from certain problems in breathing or it can be any other uneasiness. This all comes under diseases.

Health is influenced by many factors like community and personal hygiene. As we know, we live in societies, our social environment also plays a key role like-

  1. We should keep our houses clean.
  2. Proper sewage disposal system should be maintained.
  3. No stagnant water should be there.
  4. No open drains, etc. should be there.

Similarly, personal hygiene is important like –

  1. One should take bath regularly.
  2. One should wear clean clothes.
  3. Drink clean water.
  4. Eat adequate nutritious food.
  5. One should maintain social harmony and equality.


Sources of diseases

We have seen that there are many factors that can cause infection or spread disease. There can be different factors responsible for uneasiness. Let us check them out .

There are different sources of diseases, out of which the main factors are as follows-

  1. Intrinsic factors
  2. Extrinsic factors

Intrinsic factors

These are those factors that are within the human body. These diseases are caused by internal factors, therefore, they are also called organic diseases.

The internal factors can be as follows-

  1. Malfunctioning of any body part
  2. Genetic disorder
  3. Hormonal imbalance
  4. Malfunctioning of immune system

Diseases can also be caused by improper secretion of hormones like Goiter, Diabetes, Cretinism, Addison’s disease, etc.

Extrinsic factors

The diseases that are caused by factors that are not present within the body.

The extrinsic factors can be the following-

  1. pathogens.
  2. Unbalanced diet.
  3. Environmental pollution.
  4. Narcotic drugs, etc.


Types of diseases

(a)Diseases caused by pathogens


types of diseases


(b)Diseases caused by inadequate diet


types of diseases


(c) Diseases caused by environmental pollution


types of diseases


Classification of diseases

(a) Depending upon the harm it can cause to the body, the diseases fall in different categories as given below-

  1. Physical diseases that affect organs

For example: bone fracture, leprosy, etc.

  1. Infectious diseases that are spread by pathogens.

For example: typhoid, AIDS, malaria, etc.

  1. Deficiency diseases that occur due to nutrient deficiency.

For example: scurvy, Marasmus, Kwashiorkor, etc.

  1. Inherited diseases that occur due to default genetic makeup.

For example: sickle cell anemia, etc.

  1. Degenerative disease that is caused due to a gradual increase in age.

For example: poor sight, poor hearing, etc.

  1. Mental disorders that occur due to certain tensions.

For example: depression, etc.

  1. Social disorder that is due to certain bad habits.

For example: drug addiction, etc.
(b) Depending upon whether the disease can spread from a sick person to a healthy person or not.

  1. Communicable diseases
  2. Non communicable diseases

Communicable diseases: Those diseases that can spread from a sick person to a healthy person by contact are called communicable diseases. They are mostly caused by external factors. They are transmitted through direct contact or through air, water, vector, etc. They are also called infectious diseases.




For example: Common cold, Cholera Tuberculosis, Pneumonia, Malaria, etc. These diseases are mainly spread through pathogens and the mode of transmission is either air, water, vector or sexual contact. As you must have seen that sometimes, if our friend is suffering from cold and we sit close to him or use his handkerchief, we also fall sick. The reason behind this is that the virus passes to us as well and we suffer from the infection. Let us see how these factors help in the transmission of diseases.

Mode of transmission of diseases

a) Air borne infections

The diseases that are passed by air are called air borne infections. They are transferred by droplet infection.


air borne infections

For example, if any person having cold sneezes, then he releases the droplets in air. The same air is inhaled by an infected person which passes the disease causing agents to a healthy person. For example : common cold, cough, etc.


b) Water borne infections

If the infected agent is spread by dirty water, then the infection is called water borne infection.


water borne infections


For example: if the stool of any sick person passes into a water body and that water is not purified and by any chance it mixes with drinking water. This water, when taken, causes infections. For example: cholera.


c) Sexually transmitted diseases

They are caused due to Sexual contact with a sick person, use of same syringes, etc. For example: Syphilis and AIDS, etc.


sexually transmitted diseases


d) Vector borne infections

Vectors are those carriers that do not directly affect us but they transfer the disease causing agent to us.


vector borne infections


For example: Female anopheles mosquito carry malaria pathogen plasmodium in its saliva. When this mosquito comes in contact, it mixes its saliva containing pathogens with our blood. Due to which the pathogen is transferred to us. So, in the case of malaria, vector is female mosquito and pathogen is plasmodium.

Non communicable diseases

The diseases that are not transferred from sick person to a healthy person. For example: arthritis, kidney failure, etc.


non communicable diseases


For example: if a person is suffering from kidney failure or pancreas infection and you sit close to him then the disease will not be transferred to you.

(c) Depending upon the duration for which the disease lasts in us, we have different types of diseases as given

  1. Acute disease
  2. Chronic disease
acute diseases

Acute diseases

They are the diseases that last only for a short span. They do not cause such long term bad effects. For example : cold, Cough, Typhoid, etc.

chronic diseases

Chronic diseases

They are those diseases that last for a long time. They cause long term bad effects. For example: elephantiasis, diabetes, etc.

(d) Depending upon the time at which the disease is present in the body.

congenital diseases
  1. Congenital diseases
  2. Acquired diseases



Congenital diseases

These are those diseases that are present since birth. These are caused due to a genetic defect or malfunctioning of body parts or metabolic disorder etc .They are permanent. They may pass to children. For example: sickle cell anemia, etc.

acquired diseases

Acquired diseases

They are not present from childhood or we can say that they are acquired during lifespan. They can be communicable or noncommunicable. For example: Arthritis.


Symptoms and signs


Whenever there is occurrence of some disease- short term or long term, there are always certain signs of uneasiness that are felt or seen by us. Like, if somebody is infected by malarial parasite, then before the disease can be diagnosed, certain signs are seen like one feels excessively cold, high fever, loss of appetite, weakness, etc. These all signs appear during the incubation period when the infection is just spreading. They indicate the presence of disease.

The symptoms of each disease are particular. We often confuse symptoms with signs. So, let us see that what is the sign of a disease.


They actually are the symptoms that are not general or are particular for a particular disease. For example: in below given picture we can see that it can be headache, eye pain, rashes, nausea, etc.


Prevention of diseases : vaccination and immunization


If there can be diseases, obviously, we have treatments to prevent or cure them. There are very few diseases that do not have a cure. Let us see how we can prevent or cure them.

We can remove the disease by following either of two steps:

  1. Reduce the effects of disease

That is – we can take proper medication to destroy the infection causing agent in our body.


2. Remove the causes of microorganisms

That is- we can take certain measures in order to prevent the spread of microorganisms.


Prevention of diseases

1. General ways of prevention

For air borne infections

  1. Keep the house ventilated.
  2. Do not be in overcrowded places.
  3. Live in a clean environment ,maintain personal and community hygiene .


For water borne infections

Drink clean and pure water

For vector borne disorder

  1. Do not chase street dogs.
  2. Do not let water to stand near your houses.
  3. Keep food items covered.


2.Specific ways of prevention

If we have strong immune system, then the infecting agent or its toxin can’t affect us.



Suppose if you and your one or more friends are sitting close to an infected person. After a while, you feel uneasy but your friend doesn’t. This can happen as there may be a chance that your body does not possess the power to fight infections but your friend’s body may have the power to resist or fight infections. Here comes the factor of immunity. Let us learn about it.

Immunity: It is the resistance against diseases. To raise our immunity level, vaccination is done.



You must have seen that when a child is born, the doctor injects him with a few medicines. This is done in order to enhance the immunity of the newly born child. As we know, the newly born child is going to encounter a new environment, so he may take time to adapt himself. Therefore, injections provide artificial immunity to him so that he can resist himself from infections.

Vaccination: It is the process of injecting either killed or weakened microorganisms in a person’s body so as to raise his immunity level.

It functions in the following manner- when vaccine is given, that is killed or weakened microorganism is injected, the body fails to recognize it and in response, secretes certain antibodies that try or kill the infection causing foreign agent. These antibodies, once made, remain in the body and protect us throughout our life from the particular antigen.

The first vaccine was prepared by Edward Jenner in 1796 against smallpox. Now- a- days, many vaccines are given to a newborn child in order to raise its immunity.


Why do we fall ill Class 9 Question Answers


1.State any two conditions essential for good health?

Ans : Balanced diet, community and personal hygiene are the two conditions essential for good health.


2. What are the various means by which infections are spread?

Ans : Infections are spread by air, Water, Vector and Sexual contact.


3. What is immunization?

Ans : Immunization is the process of making people immune to a disease by artificial methods.


4. What precautions will you take at school to reduce the incidence of infectious diseases?

Ans :The precautions to be taken at school to reduce the incidence of infectious diseases are – drinking clean water, availability of proper food, proper ventilation and to stay away from an infected person.


5. What are antibiotics?

Ans :Antibiotics are the medicines that are advised by doctor in case of microbial infections.


6.Why are we advised to take balanced diet when we are sick?

Ans : Nourishing food is good for health and simple food and is easy to digest. It helps in strengthening the body and increases our immunity.


7.In which case would the long term effects on your health be most unpleasant?

  1. If you get jaundice
  2. If you get acne
  3. If you get lice

Ans : Jaundice will be chronic as it has long term harmful effects. It can also be fatal.


8.Under which condition you are most likely to fall sick :

  1. When you are giving exams
  2. When you are travelling in bus or train for two days
  3. When your friend is suffering from measles.


Ans : In case my friend is suffering from measles, I will be most likely to fall sick because it is an infectious and contagious disease.


9.Under what condition person is most likely to fall sick?

  1. When she is recovering from Malaria.
  2. When she has recovered from malaria and is taken care of someone who is suffering from chicken pox.
  3. When she is on four day fast after recovering from malaria and is taking care of a person suffering from chicken pox.


Ans : When a person is on a four day fast after recovering from malaria and is sitting with a person suffering from chicken pox, he is likely to fall sick soon because of less food in the body. Organs such as liver do not recover so soon. A person will get infected with chickenpox as his immunity is not too strong.


10.A baby is not able to tell his caretakers that he is sick. What would help us to find out :

  1. That the baby is sick?
  2. What is the sickness?

Ans : Signs of diseases like :Baby is cranky, restlessness is seen, baby feels hot, eyes start watering, cough, loose motions, vomiting indicate that the baby is sick.

But Sickness can be known by consulting a doctor.


11. Conduct a survey in your neighbourhood to find out what the three most common diseases are. Suggest three steps that could be taken by localites to prevent diseases.

Ans : The common diseases are: loose motions, cough and cold and malarial fever.

  1. Reduce the effect of infectious diseases.
  2. Public cleanliness needed.
  3. Immunisation.
  4. Use of clean drinking water.


12.A doctor is exposed to many sick persons than others in a community. Find out how he avoids getting sick?

Ans : To reduce the incidence of infections, the following can be done:

  1. Immunisation
  2. Wear gloves and masks
  3. Take prophylactic medicines
  4. Wearing coat
  5. Washing hands with soap after examining.


13. What changes can we make in our habits to prevent diseases?

Ans : The following changes can be made in our habits to prevent diseases-

  1. Avoid contact with infected person
  2. Drinking clean water
  3. Consume balanced diet
  4. Community and personal cleanliness
  5. Proper disposal of waste


Class 9 Science Chapter-wise Explanation