Any malfunctioning process which interferes with the normal functioning of the body is called a disease. Disease may be defined as a disorder in the physical, physiological, psychological or social state of a person caused due to nutritional deficiency, physiological disorder, genetic disorder, pathogen or any other reason.

Types of Diseases

The diseases may be classified into two broad categories:

1. Congenital disease: The disease which is present from birth (like hole in the heart in infants). They are caused by some genetic abnormality or metabolic disorder or malfunctioning of an organ.

2. Acquired disease: The disease which may occur after birth during one’s lifetime.

Acquired Diseases

Infectious diseases

The diseases which can be transmitted from diseased person to healthy person. For example, measles.

Degenerative diseases

The diseases caused by the malfunction of some vital organs of the body. For example, heart failure. 

Deficiency diseases

These are caused due to nutritional deficiency such as that of minerals or vitamins in the diet, For example, anaemia (Fe), Beri-beri (vitamin B).


This is an abnormal, uncontrolled and unwanted growth of cells. For example, breast cancer.

Important Terms

Pathogen: A living organism which causes a disease.

Parasite: An organism which gets food and shelter from host.

Host: The living body on or inside which the disease-producing organism takes shelter.

Infestation: Presence of a large number of parasitic organisms on the surface of body of the host.

Vector: It is an organism which harbours a pathogen and may pass it on to another person to cause a disease (Mosquitoes harbour malarial parasite and transmits it to humans).

Carrier: It is an organism which itself does not harbour the pathogen but physically transmits it to another person (Housefly is the carrier of cholera germs).

Reservoir: An organism which harbours pathogens in large numbers that do not cause any suffering to it.

Epidemic: Spreading of a disease among a large number of people causing a huge loss of life in the same place for some time. For example, plague.

Endemic: A disease which is regularly found among a particular group of people. For example, goitre, restricted to a certain locality or a country.

Pandemic: A disease which is found all over the world. For example, AIDS.

Interferon: Type of proteins produced by infected cells of the body when attacked by a virus, which act to prevent the further development of the same virus.

Inoculation: Introduction of antigenic material inside the body to prevent suffering from a disease.

Vaccination: Injection of a weak strain of a specific bacterium (Vaccine) in order to secure immunity against the corresponding disease. It is also called immunization.

Incubation period: The period between entry of pathogen inside a healthy body and appearance of the symptoms of the disease.

Symptoms: Specific morphological or physiological expressions which appear on the diseased organism and help in the identification of the disease.