Why do we fall ill class 9 notes | Why do we fall ill class 9 Questions And Answers

“Why do we fall ill” chapter is very important from the examination point of View. In this chapter, the students read about the conditions of being healthy, differences between the conditions of healthy and disease free, causes of diseases, types of diseases and prevention of diseases.

Why do we fall ill class 9 science notes

These notes given below have been prepared by subject matter experts having experience of more than 10 years in teaching science at secondary level. We are sure that you will love these questions and answers related to the above mentioned chapter of 9 Science.


  1. Health:- A state of complete physical, mental and social well being is called health.
  2. Physical dimension:- The physical health implies perfect functioning of all the body parts i.e., various organs and organ system.
  3. Mental dimensions:- Mental health implies harmony between the individual and its environment. A mentally sound individual is free from tension and anxiety and therefore performs his functions in a better way.
  4. Social dimension:- Human beings are social i.e., they live in groups regularly interacting with each other forming highly organized societies. Social health implies that every person living in a society possess basic requirements of life i.e., clean place of living, good earning, good food, a happy family, Cooperative interactions with neighbours, friends and leading a happy life. Health is therefore a state of being well enough to function well, physically, mentally and socially.
  5. Disease: It is a condition in which a part or organ of the body is not functioning normally.

What is difference between healthy and disease free states?

A healthy and a disease free state can be differentiated as:

HealthyDisease Free
A person with a state of complete physical, mental and social well being is said to be healthy. A person which is not having any kind of diseases is said to be disease free.
Healthy person is much more sound than the disease free person.Disease free person is not always healthy.
Behaves well in society.Does not behave well in society.

What are the various causes of diseases?

Diseases are caused by various agents. An agent may be defined as a substance or force which causes a disease by its excess, deficiency or absence.

These agents are of five main types given below:

  1. Biological Agents:- Biological agents include viruses, Bacteria, Fungi, Protozoans, helminthes. The biological agents are called pathogens.
  2. Nutrient Agents:- These comprise food components such as carbohydrates, proteins, fats, minerals etc.
  3. Chemical agents:- These include enzymes, hormones, urea, uric acid. Pollutants, spores etc.
  4. Physical agents:- These include heat, cold, sound, radiations, electricity etc.
  5. Mechanical agents:- These comprise chronic friction or other mechanical forces which result in injury, dislocation of bones or fracture.

Acute and Chronic diseases.

Acute Diseases:- These diseases last for only short period of time and are not severe. The acute diseases don’t cause long term bad effect on patient’s health. Acute diseases take little time to heal, cold, cough, typhoid etc.

Chronic Diseases:- These diseases last for long time and have drastic long term effects on j as much as life time. Diabetes, tuberculosis, cancer, elephantiasis etc. are examples of chronic diseases.

What are infectious diseases? How do communicable diseases spread from one person to another?

Infectious diseases are those diseases which spread from infected persons to others. These diseases spread in various ways i.e., through air, water, physical contact, sexual contact and insects (vectors). The causative agents of these diseases are known as pathogens or infection agents. These may be viruses, bacteria, fungi, protozoans (single celled animals) and different kinds of worms.

Means of spread of communicable or infectious diseases:-

The various means of spread of communicable diseases are:

  1. Through Air:- Microbes that cause common cold, tuberculosis, pneumonia, etc. can spread through the air. In this case when an infected person sneezes or coughs, the little droplets of water (saliva) are thrown out which contains microbes. Someone standing close by can breath in these droplets and thus microbe gets a chance to start a new infection in that person.
  2. Through Water:- Many infectious diseases spread through water e.g., cholera causing microbes enter new host when faecal matter of infected person gets mixed with drinking
  3. Through Sexual Contact:- Few infectious diseases such as syphilis and AIDS are transmitted by sexual contact from one person to other. The microorganisms responsible for causing these diseases however do not spread by casual physical contact such as handshake or hug.
  4. Through Vectors:- Many animals living with us carry the infecting agents from an infected person to other host. These animals thus act as intermediates and are termed as vectors. The vectors are therefore the carriers of the disease causing pathogens. The most common vectors are the insects e.g, the mosquito (vector) carries plasmodium which causes Malaria. The housefly (vector) carries the pathogens of cholera, typhoid, dysentery and tuberculosis on the legs.
  5. Through Physical Contact:- The pathogens also spread through physical contact or through articles of use from infected person to the healthy person.

What are Non Infectious diseases?

The diseases which remain confined to the person who develops them and do not spread to others. Non infectious disease may occur due to

  1. Malfunctioning of some important organs e.g., heart disease
  2. Malnutrition diet or deficiency of carbohydrates, proteins, vitamins, minerals e.g., Kwashiorkar, Marasmus, Beri Beri, Scurvy, Anaemia, Rickets etc.
  3. Hypo or hypersecretion of hormones and enzymes e.g., diabetes, cretinism, myxodema, gigantism.
  4. Malfunctioning of immune system e.g., allergy.

What is organ specific and tissue specific manifestation?

As compared to any type of pathogenic microbe, our body is quite large. Thus there are many possible regions, tissues or organs where a pathogenic microbe can go and stay. Different species of disease causing microbes have evolved to move and reach different tissues and organs, this is called tissue specific and organ specific manifestation.

The selection where a pathogen can go in to the body depends mainly up on their point of entry into the body e.g.,

  1. If the disease causing microbes enter with air through the nose, they are likely to go to the lungs e.g., the bacteria which causes tuberculosis of lungs.
  2. If the pathogenic microbe enter through mouth they are likely to stay in the lining of gut e.g., typhoid bacteria.
  3. Malaria causing microbes are transmitted through mosquito bite into the blood and then to the liver.

What are the principles of treatment of communicable diseases?

There are two ways of treatment for infectious (communicable) diseases, these are:-

i)To reduce the effect of disease:- It can be done by providing symptomic treatment. We can provide treatment that will reduce the symptoms which are usually because of inflammation etc. For example, we can take medicines that bring down fever, reduce pain or control loose motions. We can have bed rest to conserve our energy.

ii)Kill the cause of disease ie, pathogens:- The most common method to kill disease causing microbes is to use medicines that kill microbes. We have to choose a specific drug against particular group of microbes. Disease causing microbes are classified into different groups such as viruses, bacteria, fungi and protozoans, each of these group of microbes have some essential biochemical life processes. Our cells have different pathways than the one used by these microbes. The drugs used against these microbes block the metabolic pathways of these microbes without affecting our own. This is achieved by antibiotics.

What are the various ways of prevention of diseases?

There are two ways of prevention of diseases:

  1. General ways.
  2. Specific ways.

General ways of prevention of diseases

  • Sanitation:- Public hygiene is one of the basic key to the prevention of infectious diseases. Garbage heaps, polluted water, food exposed to dust and flies are the chief source of disease causing organisms. (Sanitary surrounding proper sanitation of our surroundings can prevent spread of diseases).
  • Eradication of vectors:- We know that vectors spread the diseases from infected person to healthy persons if vectors can be killed by suitable methods, the diseases spread by them can be controlled.
  • Sterilization:- Patient’s surroundings and articles of use should be sterilized. Soap, phenyl, dettol and antiseptic lotion may be used wherever necessary.
  • Isolation:- A person suffering from an infectious disease should be segregated to prevent the infection to spread.
  • Education:- People should be educated about the diseases so that they may protect them selves against diseases.
  • Proper and Sufficient food:- Availability of proper nutritious and sufficient food to every one will make people healthy to resist infections.
  • Specific ways of prevention of infectious diseases:- (1) Some specific vaccines have been synthesized to develop temporary or permanent immunity against a particular disease e.g. D.P.T. vaccine is used against Diphtheria. (2) Many antibiotics have been developed to fight against the disease causing microbes in an organism.

What are Vaccines?

Vaccines are generally the preparation of weakened or dead pathogens which on inoculation in a person develops temporary or permanent immunity against a particular disease in him/her by inducing antibodies formation.

In other words, we can say a vaccine is a preparation containing a pathogen either in attenuated or inactivated state. This preparation is introduced into an individual to induce adequate antibody production against the pathogen in question so that the individual becomes protected against infection, at a later date, by that pathogen.

This introduction of a vaccine in an individual is called vaccination or immunization as it leads to the development of immunity in the vaccinated individuals to the concerned pathogen.

Textual Questions of Why Do We Fall ill

In the forthcoming section, you will get the text questions of “Why do we fall ill” chapter of class 9 science and their answers. This section is very important from the examination point of view.

State any two conditions essential for good health?

The two conditions essential for good health are:

(i) Better sanitation or clean surroundings.

(ii) Availability of balanced and nutritious food.

State two conditions essential for being disease free.

Two conditions essential for being disease free are:

(i) Personal hygiene and Living in hygienic environment.
(ii) Getting vaccinated against common infectious diseases.

Are the answers to the above questions same or different? Why?

Two differ from each other as:

(i) An individual can be in poor health even being free of any cause of disease e.g., in case of social and mental health.

(ii) Health is concerned with societies and communities, while disease is concerned with the suffering individual.

List any there reasons why you would think that you are sick and ought to see a doctor. If one of these symptoms were present, would you still go to the doctor? Why or why not?

Symptoms of the disease are:

  1. Impairment of functioning of some organs of the body.
  2. Persistent coughing or loose motions.
  3. A wound with oozing pus.

If one of these symptoms were present, I would prefer to go to the doctor to get laboratory tests to confirm the disease.

In which of the following cases do you think the long term effects on your health are like to be most unpleasant : 1. If you get jaundice, 2. If you get lice, 3. If you get acne. Why?

Among the three diseases, jaundice is the disease which would have long term effects on our body. It is so because jaundice is a disease in which liver is affected. It takes long time to recover from the disease and needs proper treatment under supervision of a doctor.

On the other hand, lice can be removed easily with short treatment and so is the acne. Both these don’t produce long term affects on the body.

Why we are normally advised to take blend and nourishing food when we are sick?

We are advised to take blend and nourishing food when we are sick because blend food contains low-fibre, non spicy and easy to digest. As we lose our appetite when we are sick so it helps in digestion and provides necessary nourishment.

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

Different means by which infections diseases can spread are – air, water, sexual contact, physical contact.

What precautions can you take in your school to reduce the incidence of infectious diseases?

Infectious diseases can be reduced by public health hygiene measures. Such as

  1. Provide clean and safe drinking water.
  2. Proper sanitation and cleanliness in school premises.
  3. Awareness among students about hygienic practices.
  4. Encourage students and staff to stay at home when infected.
  5. Cover your mouth while sneezing.
  6. Proper hand wash before meals.

What is Immunization?

Immunization is a technique in which people are given particular vaccine to prevent spread of infectious disease.
Vaccines are generally the preparation of weak and dead pathogens which on inoculation in a person develop temporary or permanent immunity against a particular disease by inducing antibodies formation.

In other words, immunization is a specific way of preventing infectious disease.

A doctor / nurse/ heath worker is exposed to more sick people than others in the community. Find out how she/he avoids getting sick herself / himself.

(i)They use mask while coming close to the infected persons to protect themselves from air-borne diseases.

(ii)They get themselves vaccinated against diseases like hepatitis, typhoid, rabies, tuberculosis, etc.

(iii) Proper washing of exposed parts after coming in contact with infected persons.

A baby is not able to tell his/her care takers that he/she is sick. What would help us to find out that the baby is sick? What is the sickness?

Symptoms will help us to find that baby is sick e.g., continuous crying and restlessness of the child. Improper intake of food, body temperature and loose motion etc.

Under which of the following conditions is a person most likely to fall sick?

  1. When she is recovering from malaria.
  2. When she has recovered from malaria and is taking care of some one suffering from chicken pox.
  3. When she is one a four-day fast after recovering from malaria and is taking care of someone suffering from chicken pox. Why?

A person is most likely to fall sick in (c) condition because malaria attack has caused large scale destruction of his blood cells so is weak and anemic. Then he is on a four-day fast so is not getting proper and sufficient food which further decreases the functioning of his immune system and lastly chickenpox is a communicable and contagious disease.

Under which of the following conditions are you most likely to fall sick?

  1. When you are having examinations.
  2. When you have travelled by bus or train for two days.
  3. When your friend is suffering from measles. Why?

I will be most likely to fall sick when my friend is suffering from measles and I come in contact with him because measles is a contagious disease and also spreads by droplet infection.

Also Check: CBSE and NCERT Class 9 Floatation Notes

Also Check: NCERT Class 9 Sound notes

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