Epidemic is a derivation of two Greek word epic (upon / among) and demos (people). It is the ‘unusual’ occurrence in a community or region of a disease specific health related events “clearly in excess” of the “expected occurrence”. Thus, any disease, which occurs in numbers more than the excepted occurrence, constitutes an epidemic. It includes heart diseases, or even psychosomatic disorders.
Health affecting lifestyle like smoking, drug addiction and health related events like accidents also fall into the category of epidemics. But during disasters we are more concerned about the epidemics of communicable diseases. Having learn that epidemic is the occurrence of a particular disease in unexpected numbers, you must be thinking that who to define the excepted occurrence.
There is no designated standard number of the excepted occurrence of a disease. It varies from place to place, and region to region. A few hundred cases of a particular disease at one place can be called as the expected occurrence of the disease in that area, if this disease is common there. On the contrary where the disease is non-existent, the excepted occurrence shall be zero and in this case even a single case of that disease will be termed as epidemic. So the basis of defining an epidemic is the defining of usual prevalence of the disease in that area and this usual prevalence is called endemicity.
Let us take the example of small pox. Twenty-five years ago, small fox was quite common in our country. It was thus an endemic disease. Now it has been eradicated not only from the country but the world for that matter. It is no more an endemic disease. The excepted occurrence of small pox is zero. A single case of smallpox will, therefore, be clearly in excess of the excepted occurrence and hence would be now considered as an epidemic of smallpox. So disease, which was endemic once, may cease to be so and a single case may be taken as an epidemic.