Researchers have recently determined that the most devastating flu pandemic of modern times, the 1918 pandemic which killed at least 20 million people, had its origins in an avian virus which was transmitted to humans and then from humans directly to other humans. Pandemics in 1957 and 1968 involved viruses with avian elements. Clearly, there are reasons to worry that the current avian flu outbreak in Southeast Asia might have similar ramifications.
So far, though current avian flu virus strains have killed dozens of people, in every case the transmission was bird to person, not person to person. That could, of course, change, as it did at least three times in the 20th century.
Here are some articles:
Newsweek interview with a researcher
BBC Sept. 30, 2005
Other avian flu resources:
US Centers for Disease Control
World Health Organization
.