Swine Flu outbreak

The swine flu outbreak that has sparked widespread fear — so much so that Egypt has ordered the slaughter of the country's 300,000 pigs, even though no cases have been reported there — is easy to pin on the eponymous animal from which it emerged, but the fact is, the current epidemic is little more than an accident of evolution. If pigs are to blame, so too are birds and humans. (See pictures of thermal scanners hunting for swine flu.)

The problem begins with the wily nature of the influenza virus itself. It may be an uncomplicated thing, made up of nothing more than 10 proteins assembled into a genome that's simple even by microbiological standards, but that bare-bones genome is unusually flexible, with snap-in, snap-out gene segments that allow easy mutation and exchange of information with other viruses. That's the reason we need a new flu vaccine every year: by the time one flu season has ended and the next one begins, the virus has changed so much, it can simply shake off last year's shot. Compare that with, say, polio; the vaccine was perfected in 1955 and hasn't had to change much since.

What keeps the flu relatively in check is that there simply aren't that many species that are susceptible to it — with humans, pigs and certain kinds of birds leading the list. "There are surface markers on the cells of some species that bind with sites on the flu virus," says Dr. Peter Daszak, an emerging-disease ecologist and president of the Wildlife Trust. "The influenza virus evolved along with pigs, and it did the same with a few other mammals and with birds." (Read "To Travel or Not to Travel? A Swine Flu Dilemma.")

The adaptability of the virus, however, made it a certainty that a strain that evolved in one of the susceptible species would easily make whatever changes were necessary to allow it to survive in one of the few other eligible hosts. So quickly and efficiently does the virus transform itself that it may require just a single passage through a single individual to get that shape-shifting job done. "Different viruses from different sources enter a cell, and the virus that comes out the other end is an entirely different one," says Dr. Richard Webby, an infectious-disease specialist at St. Jude Children's Research Hospital in Memphis and the director of the hospital's World Health Organization collaborating center. "The process is called reassortment."

Birds are the natural reservoirs of the common flu strains that strike in winter — and those strains reassort themselves to hit humans particularly hard. But while humans are not susceptible to every strain of avian flu, pigs definitely are. When bird flu viruses replicate in pigs, they pick up the viral machinery that gives more selective flu strains the power to spread to other mammals, like us. That's what makes pigs such potent mixing bowls for flu. The roundabout bird-pig-human route may be less common than the straight bird-human jump, but it may be more problematic. Strains of avian flu, like the much-feared H5N1, can infect individual humans, but they can't make the person-to-person leap. Avian flu that is passed through the pig's mammalian system, however, can be passed readily among humans. (Read "Why Border Controls Can't Keep Out the Flu Virus.")

All of this made the flu virus a tenacious foe from the outset, but once humans invented farming and learned to cultivate animals, we made a bad situation much worse. All at once, chickens, ducks and pigs — which never had much to do with one another — began living cheek to jowl in high numbers and often unsanitary conditions. Farm families and people working in live markets then began mingling with the critters. That's a pathogenic speed blender, and the viruses have taken full advantage of it. "It's really an ecological issue," says Daszak.

So if we can't fairly blame the pigs (indeed, the CDC has officially stopped calling the virus "swine flu," opting instead for the more hog-friendly 2009 H1N1 flu), can we blame Mexico? That charge doesn't stick either. Decades ago, numerous countries came together to develop the Global Influenza Surveillance Network (GISN), which allows epidemiological teams to spot new flu viruses as soon as they emerge and get vaccines ready in time. But the GISN only tracks human flu, meaning animal flu can slip by undetected. What's more, pigs that carry influenza tend not to die en masse the way flocks of birds do, eliminating the immediate tip-off that a serious pathogen is at large. None of that is Mexico's fault either. In fact, since human tourists and domesticated animals cross into Mexico all the time, there's every reason to believe that the progenitor virus behind the epidemic hitched a ride in one of them.

"I'm of the opinion that this doesn't have to be a Mexico-originated virus," says Daszak. "Somehow it got to Mexico and then mixed with humans."

If we have to pin the rap somewhere then, forget any one species or country and blame simple biology. But regardless of whence the virus came, the more salient question is, Where will it go? That's what concerns doctors as they work to stem the epidemic and make sure healthy people stay that way.