Experts fear the next pandemic will start when an avian virus moves from a chicken to a person already suffering from human flu. The two viruses might then combine into a mutant strain to which humans would have no resistance. If more people get infected by birds, there's a higher risk that such a mutation will emerge. "Viruses are prone to evolve over time," warns Dr. Julie Gerberding, director of the U.S. Centers for Disease Control. "We are taking this very seriously."Is there a vaccine?No. The WHO had been working on a vaccine based on the strain isolated in Hong Kong in 1997, but the agency just announced that the virus causing the new outbreak -- though still H5N1 -- has mutated enough that vaccine development must start over. It won't be easy to make, either. Vaccines are usually produced in chicken eggs, but H5N1 is lethal to fertilized eggs. The WHO has also warned that the new strain may be resistant to some drugs that are used to fight influenza. So if this flu does spread between humans, public health officials will face new and epic challenges not only in containing the disease but also treating it. By Catherine Arnst in New York
What You Need To Know About Avian Flu
The current avian flu outbreak in Asia is the fifth since 1997 to infect humans. This has raised a red flag for infectious disease experts, who fear the strain could mutate and spark a devastating flu pandemic -- a worldwide epidemic such as the so-called Spanish flu of 1918 that killed at least 40 million people. An editorial in a recent issue of the medical journal The Lancet warns that such a threat must be taken seriously. "In view of the high mortality of human influenza associated with this strain [of avian flu]," it reads, "the prospect of a worldwide pandemic is massively frightening." Here are some of the facts about avian flu and the risk that it poses to humans.What is killing the birds of Asia?Avian flu is caused by one branch of a family of microbes called Type A influenza virus. (A different branch typically causes flu in humans.) Until recently, scientists thought people couldn't be infected by avian flu viruses, but in 1997 bird-to-human transmission took place in Hong Kong, infecting a handful of people. The viral subtype responsible for the current outbreak is called H5N1, which originates in water fowl, primarily ducks. Ducks can usually withstand the virus -- making them ideal carriers -- but chickens have little resistance. When one chicken is infected, the virus can spread to an entire flock within hours -- and soon kill nearly all the birds.How do humans get avian flu?So far, all known cases of human infection can be traced directly back to contact with live chickens. Infected birds shed the virus in their feces; humans then breathe in the fecal dust. The virus is not passed on in the meat or eggs, so there is no danger from eating cooked chicken.How deadly is avian flu to humans?Very. In the 1997 outbreak, also caused by H5N1, 18 people were infected and six died -- a 33% fatality rate. By comparison, the SARS outbreak last year infected 8,098 and killed 774, a death rate of 9.6%.Can humans infect other humans with H5N1?Not yet, but this is the possibility that has the world's leading virologists racing to Asia. Viruses are notorious shape-shifters, able to constantly rearrange their genes and swap genetic material with other influenza strains. That makes it hard to predict when and if a deadly bug will emerge. Public health officials say historical evidence suggests there are three to four flu pandemics each century. In the past 100 years there have been three, and the World Health Organization warns that "another influenza pandemic is inevitable and possibly imminent."