In Greek mythology, the mighty Lapiths had their butts handed to them by the centaurs. My question is this: what were you thinking, Lapiths? I mean, they're centaurs. They have horse bodies and man-heads. No way you're gonna beat that. Similarly, sort of, we're now confronting a hybrid little beastie of our own. The 2009 H1N1 flu virus is a crazy chimera of a thing. It is the love child of three species-specific viruses, a bird-pig-person germ. We might ask, how does this happen? And what makes it different from the regular flu? Well, I'm here to tell you something you may not know about viruses: they are promiscuous as hell. And here's something you may not know about pigs: they are little biological swingers clubs. Flu viruses don't have DNA like many organisms -- their genetic material is carried in strands of RNA. And, that RNA is divided up into little discrete segments. When viruses get to all bumpin' and rubbin' up on each other in a host cell, they freely swap these segments around. Filthy little buggers. The results of this, according to University of Chicago infectious disease expert Kenneth Alexander, is usually "¦ nothing. Most of the products of such unholy coupling are just failed viruses. But occasionally, you'll get a combination that works, and a new kind of virus survives and starts copying itself. That's where pigs come in. They have the peculiar property of being subject not just to swine flu, but also bird flu and human flu. So all those viruses can mix freely in the pig's cells, and we can wind up with a hybrid H1N1 with genes from all three variants. In this case, that variant happens to be transferrable from person to person. Unlike seasonal flu, we are now passing around a novel strain. That means none of us has been exposed before, so we don't have any acquired immunity. True, the seasonal flu tends to mutate from year to year too, but the strains are usually similar enough that we keep at least some of our past immunity. This also means that otherwise healthy adults may be at risk from pandemic flu, whereas with seasonal flu the concern tends to focus on the elderly and the very young. Seasonal flu can still do a number on you: an average of 36,000 Americans die of it every year, according to the CDC. The fear is that if pandemic swine flu were to really get out of control, and possibly grow more virulent, the mortality rate could get ugly. No sign of that for now, but if we learn one thing from the Lapiths, it's not to underestimate your enemy.