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Why do We Fall ill Class 9 notes, CBSE NCERT Ch 13 study material

Important topics covered:

  • Health and Failure
  • Healthy and Disease free
  • Diseases and its causes
  • Acute and chronic Diseases
  • Causes of diseases
  • Infectious and Non Infectious Disease
  • Means of spread
  • Organ specific and Tissue specific manifestations
  • Principles of treatments
  • Principles of preventions

Important note point of Why do we fall ill:


  • The State of complete physical, mental and social well-being of a body is health.
  • Good health comes to those who have good social and physical conditions as well as good economic conditions.
  • Clean surrounding, clean drinking water, proper garbage disposal and good sanitation are the mean of good physical and social environment.
  • A well economic condition means having a better job with good earning and eating a nutritious food thus having a well sustained life.
  • One’s personal as well as community issues both matters for health.

Community Health:

community health

  • Those activities which people do in a group as well as individually for the betterment of society, constitute the community health.
  • Personnel, as well as community health, are supplementary to each other.
  • We keep our body clean to protect ourselves.
  • We need a healthy surrounding for the same.
  • By the means of community health and development, we can achieve good health.

Distinguish between healthy and disease free:


Disease Free

When one is mentally, physically,
socially well enough to perform all
the essential function.

Disease-free is the state having no
disease at that time.

Individual, physical and social
environment all contribute equally
to it.

Only an individual is referred here.

The individual have good health.

It depends upon the condition may
have good or poor health.

Type of diseases:

Diseases are of two types. They are:

Acute diseases:

These diseases lasts for only a short period of time and affects the body quickly and suddenly.
Example: Influenza (Flu), Common cold, cough etc.

Chronic Diseases:

The diseases which lasts for a long time even for life time are called chronic diseases.
Example: Diabetes, tuberculosis, elephantiasis etc.

Difference between acute and chronic diseases:

Acute Disease

Chronic Disease 

Short duration disease

Long lasting disease

After the cure Patient recovers

Patient does not recover

Following acute disease there
is no loss of weight or feeling
of tiredness.

In chronic diseases there
is often loss of weight of
feeling of tiredness.

Short duration loss of work
and efficiency.

 Loss of work and efficiency
for a long time.

Disease and its cause (Reason for Why do we fall ill?):

  • After being affected from a disease there is improper functioning of body.
  • These live changes in the body gives rise to the sign and symptom of diseases.
  • By seeing symptoms physician tells about the particular disease and prescribes cure according to it.

Causes of disease:

The diseases are mainly caused by:

  • Pathogens like virus, bacteria, fungi, protozoans or worms causes diseases. These are called intermediate cause.
  • Lack of proper nutrients.
  • Heredity and genetic disorder.
  • Lack of proper treatment and immunization.
  • Environmental pollution consisting of water, air etc. all these are contributory cause.

Many internal and external factors are responsible for disease:

Internal factors

External factors

Hormonal imbalance

Unhealthy diet

Allergic Reaction

Disease causing germs
(bacteria, viruses, fungi)

Genetic disorder

Pollution in the environment

Malfunctioning of body organs

Unhealthy lifestyle

Infectious and non-infectious disease:

Infectious Disease:

  • Diseases which are spread by microorganism are called infectious diseases.
  • These diseases transfers from diseased man to healthy man by the action of virus, bacteria, fungi, protozoans, fungi, worms etc.

Non Infectious Diseases:

  • Diseases which are not spread from one organism to another by air, water etc. are non-infectious disease.
  • Arthiritis, Cancer, heart diseases are some examples of non-infectious disease.

Means by which infectious disease spread:

Type of Disease


Air-borne Diseases

Common cold, influenza,
measles, tuberculosis

Water-borne diseases

Cholera, typhoid, hepatitis

Sexually transmitted Diseases

AIDS, Syphilis

Animal-borne Disease


(female anopheles mosquito)

Dengue, malaria, yellow-fever

air borne diseases/ means of disease spread

Disease causing germs:

Several disease causing germs are here with their appropriate images:

Disease Causing germs

Properties and Disease


SARS Virus

It is of 500 nm in size.

SARS virus


Protozoan organism

Causes sleeping



It is of 5 micro-metre
in size

Causes acne.

Staphylococcus bacteria

Adult roundworm

Found in small

We call it ascaris

Adult roundworm


Causes Kala-azar.
It is protozoan.

These are oval

Whip like structure.

10 micro-metre in



Different micro-organism causes different diseases. These are the state behind why do we fall ill. They are:


Diseases caused by it


Common cold, influenza,
measles, chicken pox,
AIDS, Hepatitis-B etc.


Cholera, Tetanus, typhoid,
TB, anthrax etc.


Causes Skin Infections


Malaria, Kala-azar,
sleeping sickness.


Intestine infections etc.



  • The biochemical which block the pathways which is important for bacterias are antibiotics.
  • Penicillin, tetracycline are some examples of antibiotics.
  • A few bacteria builds a cell wall to protect from antibiotics.
  • The antibiotics stops the bacterial process that builds the cell wall.
  • Antibiotics act only against bacterias.
  • Antibiotics are not capable of acting against viruses as viruses do not have their own biochemical mechanisms but depend upon that of the host.

AIDS (Acquired Immuno Deficiency Syndrome):

  • AIDS is caused by Human Immuno deficiency Virus(HIV).
  • AIDS transfers from one body to another through many ways:
    Through Sexual contact.
    Blood transfusion.
    When we use razor used by AIDS infected persons.
    By using infected needle.
    AIDS can also be transferred from mother’s foetus to her child.

Prevention from AIDS:

  • One should avoid infected transfusion of blood.
  • Everyone is advised to use disposable needle and syringes.
  • One should avoid sexual relationship with unknown person.
  • In salons don’t use the used razor blades.

Principles of treatment of diseases:

To reduce the effects of the diseases

  • Effect of diseases like fever, pain, etc can be brought down by taking medicines and by taking complete rest to conserve the body’s energy.

To kill the cause of the disease i.e microbes:

  • We can treat by taking suitable antibiotics and drugs which kills the disease causing microbes.

Principles of Prevention:

There are mainly two types of cure which can prevent us from the disease causing microbes:

General method:

  • Preventing ourselves from exposure against microbes.
  • By boosting our immune system.
  • To prevent infectious disease public hygiene is most important.
  • Proper balanced diet and nutrition make us healthy and resist the infection.

By preventing to live in crowded area we can prevent us from airborne diseases.

By living in the clean environment we can prevent ourselves from vector disease.

Specific method:

  • By every disease specific measure also we can prevent disease.
  • By immunization we can prevent each specifically.

By introducing a weakened pathogen to fool the host and prepare the antibodies inside the body for that particular diseases.

The helpful thing of this method is that it not only to fight against these disease but also by remembering these incident our immune system sends antibody to our blood for future cause.

The vaccination law also follows the same.


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