Why do we fall ill Class 9 Notes

Welcome to Class 9 Why do we fall ill notes Notes for Chapter 13.The topics in this page are Overlap of personal and community issues for health ,Disease and their cause,What are the factors that cause diseases,Types of diseases,Infectious diseases,Congenital diseases and Acquired diseases. This is according to CBSE and the NCERT textbook. If you like the study material, feel free to share the link as much as possible.

Table of Contents


  • Health has been described as the state of complete physical, mental and social well being. According to the World Health Organization (WHO), health is a state of complete physical, mental and social well-being and not merely an absence of disease or infirmity.
  • For keeping a healthy life cycle, a person needs to have a balanced and varied diet, has to take exercise, lives in a proper shelter and takes enough sleep. In addition, a good hygiene tends to reduce the chances of infections.

Overlap of personal and community issues for health

  • Personal health is a state of complete physical, mental and social well-being.
  • The health of all organisms will depend on their surroundings or their environment. Human beings live in society. Our social environment therefore is an important factor in our individual health. Public cleanliness is important for individual health.
  • Community health comprises of maintaining, improving and protecting the health of entire community.
  • Social equality and harmonious relationships among our population are necessary for the individual health.
  • So there is an overlap of personal and community issue for health.
The condition necessary for good health are:
  • Step to ensure sanitation that is clean surroundings by providing good sewage and rain water disposal systems and proper garbage disposals.
  • Availability of clean drinking water.
  • Availability of adequate, nutritious food.
  • Social equality and harmony.

Disease and their cause:

  • Disease is a condition that impairs the proper functioning of the body or its parts.
  • A disease can be defined as: Any deviation from normal functioning or state of complete physical and mental well being. Note:
  • Being disease-free is a condition of absence of any kind of body discomfort. It depends on the person alone.
  • Being healthy is a state of complete, physical, mental and social well-being. It depends on the person, his physical surroundings, the society and also his or her economic status.
  • Disease has a specific cause and can be recognized by particular signs and symptoms.
  • A symptom is a change in the normal functioning of the body. A headache, fever, stomach ache, vomiting etc all are symptoms.
  • Symptoms indicate the presence of disease.

What are the factors that cause diseases?

Disease causing factors are broadly classified into two groups-
  1. Intrinsic factors
  2. Extrinsic factors

Intrinsic or internal factors:

The disease causing factors which exist within the human body are called intrinsic factors. The important intrinsic factors which affect human health are the following:
  • Malfunctioning or improper functioning of various body parts such as heart, kidney, liver, etc.
  • Genetic disorders
  • Hormonal imbalances
  • Malfunctioning of immune system of body, e.g., allergy.
  • The diseases caused by intrinsic sources are called organic or metabolic diseases.
  • For example: diabetes mellitus is caused by failure of proper insulin production and cataract is caused by opacity of lens.

Extrinsic or external factors:

The disease causing factors which enter the human body from outside are called extrinsic factors. Such as:
  • Unbalanced diet or inadequate diet.
  • Disease causing microorganisms such as viruses, bacteria, fungi, protozoans etc.
  • Environmental pollutants.
  • Tobacco, alcohol and narcotic drugs.

Types of diseases:

The human diseases are broadly classified into following four categories:
  1. Acute diseases
  2. Chronic diseases
  3. Congenital diseases
  4. Acquired diseases

Acute diseases:

Diseases such as influenza are described as acute because their effects are come on suddenly and affect the body quickly, e.g., common cold.

Chronic diseases:

Other diseases are more long-term, with the symptoms lasting for months or years. Such diseases are called chronic diseases, e.g., tuberculosis.
Note: Acute and chronic diseases have different effects on health. An acute disease, which lasts for only very short periods of time, will not have time to cause major effects on general health, but a chronic disease will affect health drastically.
For example, cough and cold is an acute disease and has no bad effect on our health but if we have tuberculosis of the lungs, then being ill over the years causes weight loss and persistent feeling of tiredness.

Congenital diseases:

These diseases are present since birth. They are caused due to genetic abnormality or due to metabolic disorders or malfunctioning of any organ. They are permanent, generally not easily curable and may be inherited to the children. For example, colour blindness.

Acquired diseases:

These diseases are those which develop after birth. Acquired diseases can be classified into two types:
  1. Infectious or communicable diseases.
  2. Non-infectious or non-communicable diseases.

Infectious diseases:

Diseases which can be transmitted from one individual to another directly or indirectly are called communicable or infectious diseases. These diseases are caused by some biological agents or pathogens such as virus, bacteria, fungi etc.
For example, tuberculosis, cholera etc.

Non-infectious diseases:

Diseases which cannot be transmitted from one individual to another directly or indirectly are called non-communicable or non-infectious diseases.
These diseases are restricted only to those persons who are suffering from them.
For example, scurvy, cancer etc.
These diseases may be of following types:
  1. Deficiency diseases: These diseases are caused due to lack of certain essential substances in our diet, e.g., protein, vitamins etc. for example, scurvy, rickets etc.
  2. Degenerative diseases: These diseases are caused due to degeneration of tissues as in old age. For example, cataract etc.
  3. Cancer is caused due to uncontrolled growth of tissue in any body parts.
  4. Allergic diseases: These diseases are caused due to hypersensitivity of the body against foreign substances, such as asthma.
  5. Metabolic disorders: These diseases are caused due to defects in the metabolic reactions.
  6. Injury: It is caused due to injury and damage to any part of the body by accidents.

Infectious diseases:

Infectious agents:
Disease causing organisms are called pathogens or infectious agents. Such as viruses, bacteria, fungi, protozoan, worms etc.
CBSE class 9 Science Why do we fall ill Notes
Why do we need to classify pathogens (infectious agents)?
  • The treatment given for a particular category does not work for other categories. However, members belonging to a particular group, say bacteria, have many similarities in structure and biochemical pathways and these are not shared by the members of other group. So, a drug that blocks one of the biochemical pathways of one group may be effective against many members of the group.
  • For example, the antibiotic penicillin inhibits processes that build the protective cell wall of bacteria. As a result, the bacteria die. Since many bacteria use such processes to build cell walls, penicillin is effective against many bacteria.
  • Antibiotics are, however, not used for killing virus, instead of that our body secrete an antiviral protein, called interferon to combat the virus of certain diseases.
Means of spread of infectious diseases:
Infectious diseases spread from one infected person to other normal person by a variety of method:
  1. Air-borne diseases: for example, common cold, influenza and tuberculosis. Some infectious diseases like tuberculosis can spread through air. An individual acquires the infection of tuberculosis by inhalation of droplets expelled through cough and sputum of the infected person.
  2. why do we fall ill notes for class 9
  3. Through contaminated food and water: for example, cholera, Hepatitis A.
    A large number of infectious diseases can also spread through contaminated food and water. Disease causing organisms and harmful pollutants in the form of chemicals can easily enter the body through food and drinks.
  4. Sexually-transmitted diseases: for example AIDS, Syphilis. Both of these pathogens are transmitted by sexual contact from one partner to the other
  5. Through vectors: for example malaria, dengue.
    any animals which live with us may carry diseases. Such animals or insects that carry infectious agents from sick person to another potential host and spread a disease are called vectors.
So means of transmission of infectious diseases may be of two main types:
Direct transmission:
  • Contact with infected person (e.g., AIDS)
  • Contact with soil (e.g., Tetanus)
  • Animal bites (e.g., Rabies)
  • Transplacental (e.g., Syphilis, AIDS)
Indirect transmission:
  • Through vectors (e.g., Malaria)
  • Through contaminated food and water (e.g., Hepatitis)
  • Air transmitted diseases (e.g., common cold, pneumonia)


Here is why dowe fall ill class 9 notes summary
  • Health is a condition of physical, mental, and social well-being.
  • An individual's health is affected by his or her physical environment as well as his or her economic situation.
  • Diseases are categorised as acute or chronic based on how long they last.
  • Disease can be caused by infectious or non-infectious agents.
  • Infectious agents are classified as creatures that can be unicellular, microscopic, or multicellular.
  • The type of treatment is determined by the category to which a disease-causing organism belongs.
  • Infectious pathogens transmit via air, water, personal touch, or vectors.
  • Disease prevention is preferable to successful treatment.
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