Why Do We Fall Ill?

Notes of chapter: Why Do We Fall Ill? are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 9.

(1) Health-

Health is a state of being well enough to function well physically, mentally and socially.

(2) The health of all organisms depends on their surroundings or their environment.

Eg:- Our health depend on cleanliness of near our surroundings.

Environment (Cyclone, heavy rain etc.) also affects our health.

(3) Disease-

Disease is a state of being uncomfortable physically, mentally and socially.

(4) When a body has diseases, the functioning of one or more systems change to bad or worse. These changes give rise of symptoms of disease like headache, cough, loose motions, wound with pus etc. Physicians give us treatment on the basis of these symptoms.

(5) Types of diseases –

(i) Acute diseases-

Acute diseases are those diseases which last for very short time period and do not harm to health.

Eg:- Common cold, fever etc.

(ii) Chronic diseases-

Chronic diseases are those diseases which last for long time period, even as much as a lifetime and do harm to health.

Eg:- Asthma, tuberculosis etc.

(iii)Infectious diseases or communicable diseases-

Diseases which are immediate caused by microbes such as virus, bacteria etc. are called infectious diseases.

These diseases are also called as communicable diseases.

Eg:- Corona, cough etc.

(a)The microbes or micro – organisms are called infectious agents.

Eg:- Viruses, bacteria, fungi, single- celled animals or protozoans, multicellular organisms (worms) etc.

The microbes can spread in the community and the diseases they cause will spread with them.

Diseases spread by microbes are listed below –

SN Microbes Diseases Characteristics of the Microbes
1. Viruses Common cold, influenza, dengue fever, AIDS Live inside host cell,

Multiply very quickly

2. Bacteria Typhoid, cholera, tuberculosis and anthrax Not live inside host cell,

Multiply very quickly

3. Fungi Skin diseases Multiply very quickly
4. Protozoan Malaria and Kalaazar
5. Worm Intestine infection, elephantiasis Multiply very slowly

(b) All bacteria are closely related to each other than to viruses and vice – versa.

This means bacteria are similar in some important life process but are not similar with viruses and vice – versa. Therefore, medicines which are prescribed for one group of bacteria likely to be work for other group of bacteria. But, same medicine will not work for diseases caused by viruses or other group of microbes.

Eg:- Antibiotics

Antibiotics are necessary for blocking of biochemical pathways important for bacteria. Bacterial make a cell – wall to protect themselves and penicillin antibiotic blocks the building of cell – wall.

Viruses do not make pathways. Therefore antibiotics are not useful in diseases born by viruses.

(iv)Non – infectious diseases-

Diseases which are not immediate caused by microbes such as virus, bacteria etc. but caused by internal factors such as genetic, physiological, behavioural etc.are called non – infectious diseases.

Eg:- Obesity, cancer etc.

(6) Causes of diseases –

(i) Drinking unclean water.

(ii) Not having nourished and proper food.

(iii) When exposed to pathogen suffer from disease.

(7) Means of spread-

(i) Communicable diseases can be spared through air by droplets of infected persons.

Eg:- Cold, cough etc.

(ii) Communicable diseases can be spared by touch of infected persons.

Eg:- Cold, viral fever, corona etc.

(iii) Diseases can be spread by drinking infected water.

Eg:- If excreta of an infected person (cholera)get mixed in drinking water and water is used by healthy person. Then person will e infected with disease.

(iv) These diseases can be communicated through sexual act.


These diseases never transmitted with casual contact as handshake, touch etc. but can be transmitted to babies by their mothers.


The animals which carry infecting agents from a sick person to another potential host are called vectors.

Eg:- Mosquitoes, flies etc.

These vectors spread malaria, dengue etc.

(8) Organ and tissues specific symptoms-

Different spices of microbes affect different organs and tissues of body.

Different microbes affected different organs and diseases spread by them with symptoms are tabulated below –

SN Entry of Microbes Microbe Organ effected Symptoms Disease
1. From air via Nose Bacteria Lungs Cough and breathlessness Tuberculosis
2. Mouth Bacteria Gut lining High fever , poor appetite Typhoid
3. Mouth Bacteria Liver Vomiting, pale stool Jaundice
4. Sexual organs Viruses Lymph nodes Swollen lymph nodes, rash, muscle pain HIV
5. Skin (Mosquito bite) Viruses Liver to red blood cells High fever, nausea, headache Malaria
6. Skin (Mosquito bite) Viruses Brain Headaches, vomiting, fits Japanese encephalitis (Brain fever)


The recruitment process in which an active immune system recruits many cells to the effected tissue to kill off the disease – causing microbes is called inflammation.

Eg:- There can be local effects as swelling and pain, general effects as fever.

(11) If the number of microbes is very small, the symptoms of disease may be unnoticed. But if number of microbes are large in number than disease can be sever of harmful to life. Our immune system should be strong because it determines the number of microbes surviving in the body.

(12)Principle of treatment –

If we get infected by the diseases we have to follow principle of treatment which has two steps.

(i) Reduce the effect of the disease by reducing symptoms that are caused by inflammation.

Eg:- We take medicines to bring down fever, reduce pain or loose motions, take rest to conserve our energy.

But this type of treatment only treats our symptoms not disease.

(ii) Kill the cause of disease by taking medicines according to type of the microbes. Because medicines of one type of microbes like bacteria will not work on other type of microbes like viruses. These medicines will kill the microbes and will save our life.

(13) Anti – bacterial medicines are easy to make than the anti – viral medicines because viruses have few biochemical mechanisms of their own. They enter our cells and use our machinery for their life processes.

(14) Limitations of the principle of treatment-

(i) Once someone has a disease, their body functions are damaged and may never recover completely.

(ii) Treatment will take time.

(iii)The person can communicate disease to other persons.

(15) Principle of the prevention-

General way of prevention from the diseases-

(i) We can prevent exposure to air borne microbes by providing not overcrowded living conditions.

(ii) We can prevent exposure to water borne microbes by providing safe drinking water after treating the water to kill any microbes.

(iii) We can prevent exposure to vector borne diseases by providing clean environment.

(iv) We should have good and proper nourishment to have good immune system that helps us to kill of the infection long before it assumes proper major proportions.

Specific way of prevention from the diseases

It relate to a peculiar property of the immune system that usually fights off microbial infections.

The process in which an individual body becomes healthy by vaccination to protect against previous microbes is called immunisation.

When our immune system first sees an infectious microbe, it responds against it and remembers it specifically. Next time same microbes or its close relative enter our body, the immune system fight against microbes with greater physical strength which is provided by vaccination. This process is called immunisation.

Eg:- Small pox, measles, tetanus, polio, diphtheria and many more diseases have vaccination. These vaccinations protect us with infecting microbes from turning into actual disease.

Helping Topics

NCERT Solutions Class 9

Worksheet Class 9



Leave a comment