Look at the history of modern global infections and you'll see a worrying pattern. For example, evidence of SARS, which killed 916 people worldwide this year, was discovered in civets and raccoon dogs sold live at Chinese food markets. Yuen Kwok-yung, head of microbiology at the University of Hong Kong, said it is "highly likely" that the virus jumped from the animals to humans.
Another disease, the Nipah virus, emerged in Malaysia in 1999 and killed more than 100 people. Scientists found that it had jumped from pigs to humans, but had originated in bats. Then there's eastern equine encephalitis -- "Triple E" -- a rare but potentially fatal disease affecting horses. It normally resides in birds. Like the deadly West Nile Virus, it is spread by mosquitoes and has jumped species to infect humans.
And the most devastating of modern diseases, AIDS, is caused by HIV, a virus that crossed from chimpanzees to humans sometime in the 1930s. Five million new infections of the virus -- the highest ever for a single year -- were recorded in 2003, and 3 million deaths. Forty million people worldwide are now infected with HIV and 15,000 more become infected each day.