Why Do We Fall Ill Class 9 Notes | Science | Term 2 |


Why Do We Fall ill Class 9 Notes | Science | Term 2 |


It is defined as the state of individuals well being enough in order to function well physically, mentally and socially. The conditions that are necessary for good health are discussed as below:

  • Proper balanced diet
  • Non contaminated food and water
  • Not being addicted to smoking, drinking or taking drugs
  • Clean and pollution free air
  • Clean, non polluted and disease-free surrounding
  • Proper hygiene
  • Regular exercise


It is that condition of the body wherein normal functioning of the body or its some part or organ is disturbed.

Difference between Being Healthy and Disease Free

Being Healthy Disease Free
It is considered as individual’s state of being well enough physically, mentally and socially. It is a state in which the body is free from any discomfort.
A healthy person will be free from all kind of diseases. It is not necessary that a disease free person will always be healthy.
It refers not only to individual but also its is social and community environment. It refers to the individual alone.
A healthy person is energetic and functions at its full biological potential. A disease free person may or may not be able to perform to its full biological potential.

Difference between Personal Health and Community Health

Personal health Community Health
It is individual based. It is community based.
Good physical, mental and social well-being of an individual are involved in it. It comprises improving, maintaining and protecting the health of the entire community.
Personal health can be maintained by taking healthy balanced diet, pure drinking water, domestic and personal hygiene, Healthy lifestyle and regular exercise. Community Health can be maintained by cleanliness of surroundings, sanitary conditions, proper disposal of sewage, providing complete Health care, clean drinking water and education about health care.

Symptoms and Signs


  • These are the changes in the normal functioning of the body system.
  • These are problem felt and noticed by patients.
  • It indicates that something wrong exists in the body.
  • It helps the doctor to diagnosis the disease but do not provide exact diagnosis of the disease.
  • Example- bodyache, headache


  • These are indications which are diagnosed by doctor.
  • It provide information of possibility of any particular disease.
  • Laboratory tests are done.
  • It provide correct diagnosis of the disease.
  • Example: High blood pressure.

Acute and Chronic Disease

Acute disease

  • These are those diseases that last only for the short period of time.
  • They last for not more than a week and do not have a long term effect on health.
  • The recovery of the patient after treatment is complete.
  • Example: Common cold, Cough, Typhoid, Malaria

Chronic Disease

  • These are those diseases that last for long time.
  • They cannot be cured completely.
  • These diseases have long term drastic effect on health.
  • Example: Kidney malfunctioning, Tuberculosis, Diabetes

Infectious and Non Infectious Disease

Infectious or communicable disease Non infectious or non communicable disease
These are caused by the attack of pathogens. These are caused by factors other than living pathogens.
These diseases are brought about by extrinsic or external factors. These disease are mainly brought by intrinsic or internal factors
The transmission of infection occurs mainly through direct contact or through some medium. This disease cannot pass from one person to another person. Transmission is absent. However, hereditary disease are transmitted from parents to offspring.
Community hygiene can effectively reduce the incidence of infectious diseases. Community hygiene is ineffective and reducing the incidence of non infectious diseases.
Example: Cold, Tuberculosis, Malaria Example: Goitre, Diabetes

Direct Transmission of Disease

Pathogens are transmitted directly from an infected person to a healthy person.
A contact with an infected person

  • When infected person comes in contact with healthy person by handshake, hugs.
  • When infected person exchange fluids with the healthy person.
  • From mother to the unborn child through placenta.

Direct : Droplet transmission

Droplets are spread during speaking and laughing by an infected person. Droplets travel some distance before falling.
Example: common cold, cough

Direct: Spread by skin

Coming in contact with infected skin
Example: chicken pox, conjunctivitis, head lice, ringworm.

Direct : Through body fluids

Body fluid or blood of an infected person come in contact with nucleus membrane / bloodstream of healthy person
Example: Semen, Blood, Saliva , Vaginal fluid

Indirect Transmission of Disease

Airborne transmission

Some infected agents remain suspended in the air and thus contaminate the air. When contaminated air is inhaled by the healthy person. Example: Measles patients are kept in isolated room.

Contaminated objects

Infected person can contaminate objects. Medical instruments like syringes can also transmit disease.

Vector borne Disease

Blood sucking insects transmit infectious agents. Insects feed on host such as animals, humans and birds. As a result the cause infection to the host.
Example: Female anopheles is a vector of malaria.

Food and drinking water

Improperly canned and undercooked food is the main source of infection. Many pathogens live inside animals and enter the human body through meat of these animals. Water can also spread infectious diseases. Water gets contaminated when infected stool mixed with that drinking water. Example: Cholera

Organ Specific Manifestation

It refers to the certain organs and tissues where the microorganisms affects the person’s body.

  • Microorganism enter the body
  • Microbes travel to the specific organ
  • Microbes multiply there

Example: Microbacterium – TB
Salmonella – Typhoid

In some cases infectious agents go to a particular organ and then spread throughout the body.

Tissue Specific Manifestation

  • In this entire tissue is affected by the microbes.
  • Immune system gets activated in response to the infection.
  • Destruction is being done by recruitment process.
  • Immune system recruits many cells to infected tissue.
  • Recruitment process is also called inflammation.
  • Local effects can also be observed is such a swelling, pain and fever.
  • It appears when the electric current effects are seen as body can no longer fight off minor infections.
  • The severity of disease is being determined by the number of microbes. The number of microbes is determined by immune system.

Principle of Treatment

There are two ways to treat an infectious Disease
Reduce effect of the disease
Kill the cause of the disease.

Bed rest are also taken in order to reduce the disease impact but this method will not make pathogens go away.

Pathogens can be killed by two common methods

  • One way is to use medicine.
  • Drug should be chosen in order to kill pathogen only.
  • Drug should not affect host’s metabolism.
  • Drugs are antibiotics.
  • Pathogenic microbes have different biochemical life process.
  • Biochemical life process could be pathways for synthesis.
  • Antibiotics block bacterial synthesis pathways without affecting our own.

Principle of Prevention

These limitation are confronted while taking an infectious diseases
In some case 100% or complete recovery is not possible due to damaged organ.
During treatment the patient is likely to bedridden for sometime.
Patient becomes a medium for spread of infection during treatment.

Public hygiene can be considered as one basic key to prevention.
Following practices are adopted for prevention:

  • Avoid exposure to airborne microbes – adopt living in crowded condition
  • Avoid exposure to waterborne microbes – safe drinking water should be provided.
  • Avoid vector borne infection- clean environment is required.

Specific ways of prevention are discussed as below:

  • With the help of strong immune system.
  • Immune system is considered as the defense system of body.
  • Immune system has specialized to cell called white blood cells.
  • Lymphocytes become active every time when pathogens enter the body.

Questions / Answers of Why Do We Fall Ill

Question 1: Why social harmony and good economic conditions are necessary for good health?

Answer: Human beings live in societies and different localities like villages or cities, which determine social and physical environment and hence both are to be kept in harmony. Public cleanliness is important for individual health. For better living condition money is required. For a body to be healthy a good food is necessary and for that purpose we have to earn. Even one has to be in a good economic condition for the treatment of disease.

Question 2: Why is the immune system necessary for our health?

Answer: The immune system of our body in acts as a defence mechanism in order to fight against pathogenic microbes. It has cells that are specialised in order to kill in affecting microbes and keep our body healthy. If the body’s immune system is strong then it can easily fight against pathogens, keeping us healthy. 

Question 3: Explain the mechanism of action of antibiotics?

Answer: Antibiotics are those chemical substances, which are obtained from some microbes which stop the growth of specific kind of pathogens. The block certain biochemical pathways which are important for the life cycle of pathogens. For example: Penicillin does not allow the formation of cell wall in some bacteria. But chemical reactions which are required for cell wall formation are blocked.

Question 4: How is the severity of disease manifestation determined?

Answer: The number of disease causing microorganisms present in the body decide the severity of the disease manifestation. If the number of microbes is quite small than the disease manifestation may be minor and may even go unnoticed. But if the number of microbes is quite large then the disease can be severe. In fact If the number of microbes is very large then the disease can even be fatal. A major factor that determines the number of microbes surviving in the body is our immune system. 

Question 5: Define the term ‘vectors’?

Answer: Vectors are those organisms which act as the intermediaries and carry the infectious agents from a sick person to a potential host are called vectors.


Why Do We Fall Ill Class 9 Science Notes

# Why Do We Fall Ill Class 9 Notes

# Why Do We Fall Ill Class 9 Questions Answers


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