Pandemics are caused by

An epidemic usually affecting a large population of the population, occurring over a wide geographic area such as a section of a nation, the entire nation, a continent or the world eg. influenza pandemics of 1918 and 1957, cholera EI Tor in 1962 and acute haemorrhagic conjunctivitis in 1971 and 1981 Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 101