In order to understand the occurrence of diseases in more than the excepted number and why there are greater chances of spread of an epidemic during a disaster we need to understand how diseases occur.

Diseases occur as a result of interaction between an agent, a host and the environment. Under the normal conditions, there is a stage of equilibrium among these but in unfavorable conditions this equilibrium gets disturbed and diseases occur in human body.

Let us understand the terms and environment before we learn about causes and characteristics of an epidemic.


A disease ‘agent’ is defined as a substance living or non-living the excessive presence or relative lack of which may initiate the disease process in man. Examples of living agents are: bacteria, viruses, fungi and protozoan whereas nonliving agents are nutrients, chemical substances and physical forces such as heat, coal and pressure.



Host is the organism in which diseases occur and for us man is considered as host for all practical purposes. A number of host factors such as age, sex, nutritional status and socio-economic factors are responsible for occurrence of diseases. In epidemiological terms, man is also defined as the ‘soil’ and disease agent as ‘seed’.


Environment is a set of conditions under which human beings live and can be defined as “all that which is external to individual human host living or non-living and which he is in constant interaction”. This includes all of man’s external surroundings such as, air, water and sanitation.