What Is a Pandemic?
Confused about what a pandemic is versus an epidemic? WebMD explains.
The Next Pandemic
The world is closely watching a virus known as avian influenza H5N1, or
"bird flu." Don't confuse it with pandemic flu. It isn't one. At least,
it isn't one yet.
At this point it's known that people have caught the virus from sick
poultry, and that the virus is very deadly to people who are infected.
Scientists worry that at some point the H5N1 virus will mutate into a form that
can pass from human to human, which it cannot do at present.
"If it adapts to a strain that's contagious among humans it will no
longer be a bird virus. It will become a human influenza virus," Epstein
Then, if this hypothetical strain is able to pass easily between people, it
may become a pandemic flu.
"It's impossible to predict whether this virus will mutate enough to be
easily passable from human to human," Pearson tells WebMD.
Another flu pandemic is almost a certainty. But an entirely different virus
may cause the next pandemic. It will not necessarily develop from H5N1.
The Flu's History
The three pandemics of the 20th century were caused by what are known as
"type A" flu viruses. It's possible that a type A virus that's in
circulation among humans today may change into a new strain that's very
contagious. Then we might have a pandemic.
The CDC keeps track of the influenza strains that circulate widely in the
U.S. each year. In the 2004-2005 flu season, the dominant strains were
influenza type A (H3N2) and influenza type B viruses. A version of the virus
responsible for the 1918 pandemic, type A (H1N1), also circulated.
The World Health Organization (WHO) constantly monitors flu cases throughout
the world, relying on information from a wide network of sources, including
government health agencies, university scientists, and international aid
WHO has developed a system of identifying where the world stands with regard
to pandemic flu. The system has six phases:
Phase 1 -- No new influenza virus has been found in people
Phase 2 -- New virus has appeared in animals, but no human
Phase 3 -- A new strain of animal influenza virus infects
humans, but there have not been human-to-human infections.
Phase 4 -- The new virus passes from person to person, but
transmission is limited and confined to a certain location.
Phase 5 -- There is frequent transmission of the virus
between people in a particular place, but it hasn't spread to the rest of the
Phase 6 -- Pandemic. The virus is widespread