》LIVES IN ASIA IS NO ORDINARY FLU [转摘《NATIONAL GEOGRAPHIC》]

王升按——本文是《国家地理》英文版杂志有关禽流感的文章，只是选段。上面一篇是译文，我将翻译此杂志上有关禽流感的系列文章。
The Spanish flu virus had recently crossed into people from some unknown animal,leaving victims with little immunity to this new threat.

ONE REASON you generally get over the flu after a few day's discomfort is that your immune system has seen it before and knows how to respond.This year's bug won't be a carbon copy of last year's,because the virus mutates constantly.But it will look similar enough that your body can almost always keep it in check.

Every so often,though,something new comes along from that animal world——a vast preserve of type A flu viruses,the ones that cause the most serious illness in humans.In far-flung studies in the late 1960s and 1970s,from Australia's Great Barrier Reef to lakes in northern Canada,Rober Webster and his colleagues tracked flu to its source."Where do flu viruses come from?"he asks,"From the wild birds of the world,the wild aquatic birds—— the waterfowl,the ducks,the shorebirds."

Dozens of flu subtype iwhabit the birds' guts,mostly harmless to their hosts or to any other creature.But occasionally one infects domestic poultry.Even more rarely,a bird virus or some of its genes slip into the much smaller pool of type A viruses that infect humans.Normally a flu virus good at infecting birds can't attack humans because it isn't equipped to invade and grow in human cells.Until recently scientists thought avain viruses could gain that ability only by indulging in the viral equivalent of sex.Because flu viruses carry their genetic information on eight separate RNA segments,it's easy for different subtypes to swap genes if they happen to meet.The result——offspring with new abilities.

For an avain flu and human flu to mix it up, they have to infect the same animal. Scientists have long considered the pig a likely mixing vessel, because pig cells have surface molecules that allow entry to both kinds of virus. A pig could conceivably catch a human flu from a farmer and a bird virus from, say, ducks at the same farm. The two viruses could then "reassort", creating a hybrid that——in the worst case——would now be able to infect human cells while still carrying bird-virus genes that would make it radically new to the immune system of the people who catch it, and unusually virulent.

Reassortment explains the two lesser flu pandemics of the 20th century, in 1957 and 1968. In each year a new flu subtpye appeared, combining genes from the human virus that had been causing mind outbreaks in prior years with new genes from a bird virus. The new pandemic viruses raced around the world, together killing about two million people.

But in 1918, Taubenberger now believes, something different happened. "We think it's pretty likely that the virus was not derived from a previously cirulating human virus", he says. All of its genes make it as an animal virus, pure and simple, that some how crossed to people without the help of genes from a previous human strain.