Countries in Asia are rushing to contain a lethal, bird flu virus that spreads to humans from chickens. The outbreak comes a year after the emergence of another deadly virus in Asia - severe acute respiratory syndrome, or SARS, a pneumonia-like illness that has killed hundreds of people worldwide. Experts say there is no connection between the two diseases.
SARS is caused by a mutated cold virus, known as a corona virus, that is harbored in wild animals, notably civets, a cat-like animal that is a delicacy in China. The bird flu is caused by a particularly aggressive virus carried by chickens.
David Heymann headed up the World Health Organization's SARS response unit at the height of the outbreak last year. While the bird flu is an exception, Dr. Heymann says most viruses that are transmitted from animals to humans are not harmful.
"They do not seem to transmit easily from human to human, the exception being, of course, influenza," he said. "And that is our great fear, that we will have shortly a major epidemic of a new influenza, which will cause high mortality and for which we are not prepared."
Some public health officials worry that the bird flu that's sickened and even killed some people in Asia, and infected millions of chickens could turn into an epidemic.
David Swayne of the U.S. Department of Agriculture's Southeast Poultry Laboratory in Athens, Georgia, says avian viruses are common in wild birds, and occasionally they spread to domestic poultry such as chickens.
When that happens, Dr. Swayne says the bird viruses can find their way into humans. He says a human outbreak of influenza takes place when proteins from the two viruses combine to form an entirely new influenza strain that's not recognized by the human immune system.
"And then if you had the population at risk, did not have antibodies against, say, the new virus, then because you have human-to-human transmission and you have a very susceptible population, then you could result in a very severe epidemic," he said. "And sometimes the word used is a 'pandemic.'"
Officials stress the bird flu is not an epidemic or pandemic at this point, since the people who have become sick appear only to have come in direct contact with infected chickens in unsanitary conditions, not other people. The virus thrives in the birds' digestive and respiratory tracts.
As a safety precaution, millions of chickens infected with the bird virus are being slaughtered, a measure Dr. Swayne says could severely impact the region.
"The avian influenza outbreak that is now ongoing in Asia, because it is a highly pathogenic virus for poultry, it does impact international trade," he said. "So when a country indicates they do have the virus, generally the first step is all international trade of poultry or poultry products are generally halted."
China, South Korea, Japan and Vietnam are among those countries that have banned chicken imports.
In the meantime, scientists are working quickly to try to develop a vaccine to protect humans from the bird flu.