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At what stage is the spread of the H7N9 virus considered an epidemic?

Question submitted by Patrick Carlson, from United States, April 29, 2013

Answered by: Laurie Garrett, Senior Fellow for Global Health

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The H7N9 virus, a new strain of the bird flu in China, has so far claimed twenty-two victims and officials are watching its spread closely. Most worrying, at least one case has turned up in Taiwan, and three of the victim's healthcare workers have developed flu symptoms, suggesting both that this epidemic has crossed country borders and that it can pass from person to person. Even more distressing, the healthcare workers wore full protective gear.

There are three stages to infectious disease spread, and the boundaries between them are a bit fuzzy. The first, outbreak, starts with the initial case(s) in people or animals, typically as a virus mutates or exploits a new ecological opportunity to infect a broader range of species. An example of this is the H5N1 avian flu, which periodically breaks beyond birds to infect (and often kill) humans.

The term epidemic is applied if that virus (or bacterium) manages to spread from person-to-person for two "generations." If John passes his virus to Jill, that is one generation of transmission; if Jill then passes infection to Mary, that's a second generation. Public health officials usually grow especially alarmed if a third generation of spread occurs, as that indicates genuine contagion is unfolding. Examples of this would be the 2003 initial Chinese SARS epidemic, and this month's bird flu in China.

Finally, a pandemic is an epidemic that spreads from one continent to another, threatening the entire world. The so-called swine flu, caused by H1N1 influenza in 2009 would be an example of this. The World Health Organization tried to create a standard pandemic alert system, but the biology of outbreaks and microbes defies simple algorithms.

One can only hope that the new H7N9 virus is not a new pandemic being born, but we will have to wait and see.