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Scientists suspect that a similar species jump--from birds, to pigs, to humans--caused the infamous flu outbreak that killed an estimated 50 million people in 1918-19.

We are long overdue for a really big influenza pandemic, Curson said. The last was over 30 years ago, and it is probably only a matter of time before something like the present bird flu combines with human flu into something really virulent.

About 20 new diseases have cropped up since the 1970s, and scientists expect this pace to continue, if not accelerate.

Much of the blame lies with misguided factory farming practices, experts say, like the overuse of antibiotics in animal feed--increasing the chance of antibiotic-resistant bacteria--and the feeding of meat products to herbivores, provoking a gruesome human variant of mad cow disease.