Pandemics are states of disease that sharply increase in populations around the world with infections taking place more or less simultaneously.
Pathogen transmission through a population is typically covered by five general descriptions.
An endemic infection is one that remains relatively stable over time, infecting an expected number of hosts in ways that are usually fairly well understood. The parasite schistosomiasis, for example, can cause serious infections, but is usually contained within tropical regions in numbers that don't vary much from year to year.
An outbreak describes a sudden spike in transmissions in a localised region. For example, in 2019 the Democratic Republic of the Congo saw a steep rise in people contracting the Ebola virus in the nation's east. While the World Health Organisation (WHO) viewed it as a public health emergency, its containment meant it wasn't an epidemic.
Widespread outbreaks across wider regions tend to be regarded as epidemics. The spread of Ebola across Western Africa between 2013 and 2016 is often described using the word epidemic.
Once the epidemic has proven to be capable of moving around the world in a way that sustains widespread, ongoing infections, it can be regarded as a pandemic.
When does an epidemic officially become a pandemic?
In 2009, a new kind of influenza A virus known as H1N1 emerged in an outbreak in the United States. It quickly spread, and was defined by the WHO as a pandemic based on a formal process that took into account the precise nature of the countries where an infection was diagnosed.
Concerned premature use of the term could cause panic, officials instead described the global epidemic as having "pandemic potential", while claiming that they weren't yet observing the "uncontained global spread" of the virus.
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