**1.1 Epidemic disease**

The term epidemic is derived from Greek word "epi" meaning "upon" and "demos" meaning "people". It refers to a communicable disease which spreads rapidly in a given population within a very short period of time. Any infectious disease existing in a region does not make it epidemic unless it causes faster mortality. A death rate of around 1.6 folds higher than usual death rate (baseline) caused by a

disease in a population within a fixed period could be considered as an epidemic disease. A disease lower than this fold increase, observed in a population could be designated as an outbreak of a disease [1].

Diseases like tuberculosis, hepatitis, yellow fever, chikungunya, ebola virus disease, marburg virus disease, Crimean-Congo haemorrhagic fever, rift valley fever, typhoid fever, Shigellosis, plague, lassa fever, West Nile fever, zika virus disease, meningitis, MERS-CoV, plague, monkeypox, nodding syndrome, nipah virus infection are considered as epidemic diseases as per World Health Organization [2]. Epidemic diseases like plague, small pox and cholera caused unsurpassed deaths in human population till the end of eighteenth century [3].
