The term pandemic, (derived from Greek words 'pan' meaning all and 'demos' meaning people) is an epidemic of infectious diseases that spreads through populations across a large region, for instance a continent, or even worldwide.

According to the WHO, a pandemic can start when three conditions have been met. Firstly, it should be an emergence of a disease new to a population. Secondly, the agents infect humans, causing serious illness. And thirdly, agents spread easily and sustainably among humans.

The last pandemic the world saw was the Hong Kong flu, which killed an estimated one million people worldwide during 1968-69. The pandemic infected approximately 500,000 Hong Kong residents, 15 percent of the population, with a low death rate. In the United States, approximately 33,800 people died.