Word: viruses
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...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...
...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...
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...
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...
...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...