Disease can be defined as the state of human health which is not at ease is not comfortable. During disease, the functioning or appearance of one or more systems of the body changes.

Classification:

(a) Based on duration:

Acute diseases: Diseases that last for only short period of time.

Example, headache, common cold etc.

Chronic diseases: Diseases that last for long time is called chronic disease. Example, tuberculosis.

(b) Based on cause: Disease can be grouped as infectious/communicable disease and non-infectious or non-communicable disease.

Infectious diseases: These diseases are caused due to microbes and can spread from one person to another.

Non-infectious diseases: These type of diseases do not spread in the community, but remains internal.

Example, cancer, genetic abnormalities.