A pandemic is an epidemic disease that has spread across populations over vast areas such as continents or even globally.

A pandemic is not to be confused with an endemic, which describes illness that is contained within one population, without intervention being required. An endemic therefore refers to disease that can be kept under control with regard to how many people it can infect. Pandemics on the other hand, have posed the most fatal threats to mankind throughout history, having claimed the lives of more people than all accidents and wars combined.

Two examples of well-known pandemics are tuberculosis, which is an airborne bacterial infection, and smallpox, a viral infection that has affected humans for thousands of years. More recent examples of pandemics include human immunodeficiency virus (HIV) and the H1N1 pandemic of 2009.

The World Health Organization (WHO) uses a six-stage classification system to describe how influenza changes from a disease that infects a few people to one that has become pandemic. Initially, the virus mainly affects animals, with a few cases of animal to human transmission occurring. The virus then starts to get transmitted between people and eventually becomes a new virus that is highly infective and out of control, with the potential to spread across the world.

A health condition can only be described as pandemic if it is infectious and the term does not apply to a disease or condition solely on the basis that it is widespread or fatal. Cancer, for example, claims millions of lives every year but as a non-transmissible illness, it is not referred to as a pandemic.