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Factors affecting health

Symptoms of disease

Signs of disease

Types of disease based on duration

Effects of acute and chronic diseases

Causes diseases

Agents of infectious diseases

Methods of Transmission

Direct modes of transmission

Indirect modes of transmission

Diseases by agents

A few diseases, agents and modes of transmission

Organ-Specific and Tissue Specific Manifestations

Principle of Treatment

Biochemical life process or pathways

Principles of prevention






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

Distinction between “Healthy” and “Disease-free” condition

The word disease means being uncomfortable due to a particular cause.

Poor health does not always mean that we are suffering from a particular disease.

Good health has different meaning for different people. It really means being able to do things, which make us happy, well.

For example for a dancer, good health means being able to dance and pose gracefully.

One can be in poor health without there being a simple cause in the form of an identifiable disease.

Health is concerned with societies and communities.

Factors Affecting Health

Our health depends on our surroundings or environment. The environment includes physical, social and economic environment.

Physical environment: For example, health is at risk in a cyclone in many ways

Social environment: Our social environment is an important factor in our individual health. We live in villages, towns or cities. In such places, even our physical environment is decided by our social environment.

If there is garbage thrown in our streets, or if there is open drain, water lying stagnant around where we live, the possibility of poor health increases. Therefore, public cleanliness is important for individual health.

Economic Environment: Food needed for good health has to be earned by doing work. For this, the opportunity to do work has to be available. Good economic conditions and jobs are therefore needed for individual health.

Social quality and harmony: We need to be happy in order to be truly healthy. If we mistreat each other and are afraid of each other, we cannot be happy or healthy. Social quality and harmony are therefore necessary for individual health.


Symptoms of disease are the things we feel as being ‘wrong’. For example headache, cough, loose motions, wound with pus etc. are all symptoms. They can only be perceived by the patient.

Sympoms indicate that there may be a disease, but they don’t indicate what the disease is.

There are various organs and organ systems in our body that carry out specific functions. In case of any disease, the functioning or appearance of one or more organs or systems of the body, changes which give rise to symptoms of the disease.

For example, a headache may mean just examination stress or, very rarely, it may mean meningitis, or any one of a dozen different diseases.


Signs of disease are objective evidence of a disease, like a skin rash and which can be recognized by the doctor, family members, and the patient. Signs give a little more definite indication of the presence of a particular disease.

Physicians will also get laboratory tests or diagnosis done to pinpoint the disease further.

Types of disease based on duration

(i) Acute diseases: Diseases that last for only very short period of time, are called acute disease, e.g. common cold.

(ii) Chronic diseases: The diseases, which last for a long time as much as lifetime, are called chronic diseases, e.g. elephantiasis.

Effects of Acute and Chronic Diseases

Any disease that causes poor functioning of some part of the body will affect our general health as well. This is because all functions of the body are necessary for general health.

However acute and chronic diseases have different effects on our health.

An acute disease, which is over very soon, will not have time to cause major effects on general health, while a chronic disease will do so.

For example, a cough and cold, which lasts for around a week, will not cause any permanent bad effects on our health. But if we get infected with a chronic disease such as tuberculosis of the lungs, then being ill over the years does make us lose weight and feel tired all the time.

Chronic diseases lead to prolonged general poor health and very drastic long-term effects on people’s health as compared to acute diseases.

Causes of diseases

All diseases have immediate and contributory causes.

Immediate causes of disease are of two types:

(i) Infectious causes: These include microbes or micro-organisms. Diseases where microbes are immediate causes are called infectious or communicable diseases.

(ii) Non infectious causes: The immediate causes of disease is not external (microbes). The causes are internal and non-infectious. Such diseases are called non-infectious diseases, e.g. cancer is caused due to genetic abnormalities.

The method of treatment is different for infectious and non-infectious diseases.

Agents of infectious diseases

Disease causing organisms include viruses, bacteria, fungi, protozoans, worms etc.

Methods of Transmission

Direct modes of transmission

Direct transmission refers to the transfer of an infectious agent from an infected host to a new host, without the need for intermediates such as air, food, water or other animals. Direct modes of transmission can occur in two main ways:

Indirect modes of transmission

Indirect transmission: when infectious agents are transmitted to new hosts through intermediates such as air, food, water, objects or substances in the environment, or other animals. Indirect transmission has three subtypes:

Diseases by Agents

Bacterial diseases - typhoid fever, cholera, tuberculosis and anthrax

Viral disease - common cold, influenza, dengue fever, chikungunia, swine flu, and AIDS/HIV

Diseases caused by Protozoa: Malaria - plasmodium and kalaazar - leishmania

Desease caused by Worms – Elephantiasis,

A Few Diseases, Agents and Modes Of Transmission


Cause (Type of organism)

Mode of Transmission

Common cold

Virus (Rhinovirus and others)


Swine Flu

Virus (H1N1, H1N2 etc.)

Direct contact


Virus (Dengue virus  [DENV])

Female Aedes aegypti Mosquito


Virus (Chikungunya virus  [CHIKV])

Aedes aegypti and Aedes albopictus mosquitoes


Bacteria (Salmonella typhimurium)

Food or water


Protozoa (Plasmodium)

Female Anopheles Mosquito

Kala Azar

Protozoa (Leishmania)

Female sandfly


Protozoa (Entamoeba)

Fecal-oral route, contact with dirty hands


Bacteria (Streptococcus pneumonia)



Bacteria (Mycobacterium tuberculosis)



Bacteria(Vibrio cholerae)



Bacteria (Neisseria gonorrhoeae)

Sexual contact


Bacteria (Treponema pallidum)

Sexual contact


Virus (HIV)

Blood, saliva, sexual contact

Sleeping Sickness

Protozoa (Trypanosoma brucei)

Tsetse files


Bacteria (Staphylococci)

Genetics, lifestyle

Severe acute respiratory syndrome (SARS) 

SARS Virus

Direct contact, Air

Elephantiasis (lymphatic filariasis)




Unknown, Weak immune system

Organ-Specific and Tissue Specific Manifestations

Different species of microbes attack different parts of the body. This depends on their point of entry.

If they enter from the air via the nose, they are likely to go to the lungs. For example bacteria causing tuberculosis goes to lungs.

If they enter through the mouth, they will go to the gut lining like typhoid causing bacteria, or go to the liver, like the viruses that cause jaundice.

But this needn’t always be the case. An infection like HIV; that comes into the body via the sexual organs, will spread to lymph nodes all over the body.

Similarly, Malaria-causing microbes, entering through a mosquito bite, will go to the liver, and then to the red blood cells. The virus causing Japanese encephalitis, or brain fever, will similarly enter through a mosquito bite. But it goes on to infect the brain.

The signs and symptoms of a disease will depend on the tissue or organ which the microbe targets.

If the lungs are the targets, then symptoms will be cough and breathlessness.

If the liver is targeted, there will be jaundice. If the brain is the target, we will observe headaches, vomiting, fits or unconsciousness.

Other effects depend on the fact that the body’s immune system is activated in response to infection. An active immune system recruits many cells to the affected tissue to kill off the disease-causing microbes. This recruitment process is called inflammation. There are local effects such as swelling and pain, and general effects such as fever.

In some cases, the tissue-specificity of the infection leads to very general-seeming effects. For example, in HIV infection, the virus goes to the immune system and damages its function. Thus, many of the effects of HIV-AIDS are because the body can no longer fight off the many minor infections. In such case small cold can become pneumonia and a minor gut infection can produce major diarrhoea with blood loss. Ultimately, it is these other infections that kill people suffering from HIV-AIDS.

The severity of disease manifestations depend on the number of microbes in the body. If the number of microbes is very small, the disease manifestations may be minor or unnoticed. But if the number is of the same microbe is large, the disease can be severe enough to be life-threatening.

Principle of Treatment

The treatment will vary depending on the microbe which is the cause of disease. They are viruses, bacteria, fungi or protozoa.

This is because each of these groups of organisms has some essential biochemical life process which is peculiar to that group and not shared with the other groups. These processes may be pathways for the synthesis of new substances or respiration.

These pathways will not be used by humans.

For example, bacteria build a cell wall. If we can find a drug that blocks the bacterial synthesis pathway without affecting our own, we can kill and control the growth of bacteria. This is achieved by the antibiotics.

Similarly, there are drugs that kill protozoa such as the malaria parasite.

The diseases caused by bacteria are treated with medicines called antibiotics, whereas viral diseases cannot be treated with antibiotics.

Viruses have few pathways or biochemical mechanisms of their own and they use the host cell to multiply. That is why there are relatively few virus-specific drugs. Despite this limitation, there are now effective anti-viral drugs, for example, the drugs that keep HIV infection under control.

The first antibiotic penicillin was extracted from fungi Penicillium notatum by Alexander Flemming in1944.

Principles of prevention

Prevention of diseases is better than their cure.

(i) General ways: General ways involve preventing exposure.

For air-born microbes we can prevent exposure by avoiding overcrowded places.

For water-borne microbes we can prevent exposure by providing safe drinking water.

For vector-borne infections we can provide clean environment.

(ii) Our immune system also plays an important role in killing off microbes. Proper nourishment is necessary for better functioning of our immune system. For this one should take balanced diet, including carbohydrates, proteins, milk, vitamins, fruits and vegetables etc.

(iii) Specific ways: Suffering from a disease once is a means of preventing subsequent attacks by the same pathogen. In case of any infection for the first time our immune system responds against it specifically. Next time when the same microbe enters our body the immune system responds with greater vigour which eliminates the infection more quickly than the first time.

Immunisation is done to prevent diseases. It involves vaccination.

Vaccines are made of dead or weakened pathogens. When these are introduced in the body, they create anti-bodies. When the actual diseases causing pathogens enter the body, these antibodies attac and kill them.

Vaccines to prevent many diseases are now available under the public health programme of childhood immunisation for preventing infectious diseases run by the government.

Children are vaccinated against tetanus, diphtheria, whooping cough, measles, polio etc.