H1N1 and its descendents
In a typical year, about 10% of Americans get the flu.But already, 2009 is not a typical year. We're in the midst of a flu pandemic caused by a virus that first emerged in Mexico in mid-February.
So far, the 2009 pandemic has been more widespread than lethal. But it's difficult to make firm predictions about flu pandemics for a number of reasons. Flu viruses are notorious for being genetically unstable, so the course of a flu pandemic may swerve midstream if the virus that's causing it mutates.
Immune responses are also hard to predict. Part of what makes a pandemic possible is that large numbers of people don't have immunological protection against a new infectious agent. But sometimes a new virus is close enough to those circulating in previous years that many people's immune systems manage to fight off the newcomer.
On the surface
We know influenza is caused by viruses. Scientists have categorized flu viruses into three main groups — A, B, and C. The viruses in the A group are the big troublemakers: common and capable of infecting people, other mammals, and birds. The new pandemic virus — and, indeed, most of the flu viruses you'll ever hear about — is in the A group.
By longstanding convention, flu viruses are further categorized and named by two proteins they brandish on their outer coats. One is hemagglutinin, which latches on to cells and infects them — it's the way the virus gets its foot in the door. The other is neuraminidase, a different surface protein that lets the virus's progeny escape once it has reproduced. Only the initials, H and N, and numbers are used in flu names. To date, only three combinations — H1N1, H2N2, and H3N2 — have been found on viruses that spread from person to person and therefore are capable of causing human epidemics. The virus causing the 2009 pandemic has the H1N1 combination.
Bird flu takes off
For the most part, birds and flu viruses coexist. The viruses may sometimes cause the birds little harm: they just live quietly, swapping genes and mutating inside the birds' digestive systems.
But occasionally, these bird (or avian) flu viruses change in such a way that they are capable of infecting other species. Researchers now believe that the 1918–19 flu epidemic was caused by an avian flu virus that not only jumped from bird to human, but also adapted to the human respiratory tract, enabling easy person-to-person transmission. This was the original H1N1 virus and the ancestor of so many others to come.
Aside from a 20-year hiatus between 1957 and 1977, different versions of human-adapted H1N1 have been circulating in people ever since. Subsequent flu pandemics (in 1957 and 1968) have been caused by viruses with genes that were a mixture of those from H1N1 and other bird flu viruses.
In 1997, a new avian flu virus, H5N1, emerged in Hong Kong. The outbreak was contained, but until the 2009 pandemic, H5N1 had been the big worry. As a new avian flu virus with which humans have no immunological experience, H5N1 has potential to cause a human pandemic like the one in 1918–19. And it's been lethal, killing over half of the people it has infected. But so far, it's infected only a few hundred people and hasn't been very transmissible from human to human. The "so far" needs to be stressed. H5N1 could change to become contagious. By the same token, the new H1N1, which has been very contagious but not very lethal so far, might undergo a genetic change and become deadly.
Pigs mix it up
The H1N1 avian flu virus that found a new home in the human respiratory tract in 1918 also set up shop that year in the airways of pigs. For the next six decades or so, "classical," swine-adapted H1N1 virus circulated in pigs in this country, with only occasional infection of people, although pig-to-person transmission probably happens more often than recognized because the symptoms of swine and regular flu are similar.
A new "triple reassortant" virus that combines genetic elements of the avian, human, and swine viruses has been circulating in pigs in the United States since about 1999. Pigs in Europe and China have been infected with a different virus — swine-adapted of avian origin — since about 1979.
The new H1N1 flu virus seems to be a mixture of some genes from the triple reassortant virus and some from the Eurasian swine-avian one.
So very typical
Symptoms of this pandemic flu are like those of the regular flu: cough, fever, muscle aches, sore throat, and so on. Nausea and vomiting may be a bit more common, fever a little less. So if you get the flu come fall or winter, it may be hard to tell just by symptoms if you've been infected by one of the regular seasonal flu viruses currently in circulation or the new pandemic H1N1 virus.
The pandemic flu also seems to be transmitted in much the same way as garden-variety flu: infected people expel the virus in respiratory droplets when they cough or sneeze. One big difference between the regular seasonal flu and the pandemic flu is that the pandemic flu seems to be hitting younger people, pregnant women, and obese individuals harder than the regular flu does. Younger people may be more vulnerable than older people because older people might have some immunity to the new virus "left over" from exposure to similar H1N1 viruses that circulated years ago.
Six flu-fighting tips
November 2009 update