In the first warning to U.S. citizens about bird flu (search), the government urged doctors Tuesday to ask patients with flu-like symptoms if they have traveled to places in Asia where bird flu has broken out.

The goal is to prevent the possible human spread of the virus, although there have been no documented cases of this occurring during the Asian outbreak.

"We are taking this very seriously right now," said Dr. Julie Gerberding (search), head of the Centers for Disease Control and Prevention (search) in Atlanta.

No cases of bird flu in either people or birds have been seen so far in the United States.

Gerberding said if doctors find that patients with flu symptoms have been in Vietnam or other places with bird flu, they should test for the virus. If it turns out they have influenza A, then state and local health departments working with CDC can check further to see if it is the H5N1 bird flu strain.

Gerberding said people who might have the flu should voluntarily tell doctors about their Asian travels if no one asks them about it.

"I want to emphasize that right now it appears to be very unlikely, but we want to be very vigilant and to make sure we are doing everything we can to detect any possible introduction here," Gerberding said.

She urged people who travel to areas with outbreaks to stay away from poultry farms, live animal markets or any surfaces contaminated with bird droppings.

So far, H5N1 bird flu has been found in eight Asian countries, and eight people are known to have died from the infection. While all appear to have caught the virus from chickens, experts suspect the disease can be passed person to person, since that occurred during a similar outbreak in Hong Kong in 1997.

The ultimate fear, though, is that someone will catch the bird flu while already infected with the ordinary human flu virus. The two viruses might swap genes, creating a mutant that could spread easily with disastrous results.

Considering the millions of chickens infected with the virus, spread to humans appears to be rare. Gerberding said there is no indication so far for a travel alert or advisory for Americans going to areas affected by the outbreak.

Last year's SARS outbreak was the last time U.S. doctors were told to be on special alert for the introduction of a contagious respiratory virus from abroad. Typically flu is much more contagious than SARS, so containing a human outbreak might be much more difficult than stopping SARS, which health officials did by isolating infected people.

Gerberding said the CDC is reviewing its infection control guidelines to be used "in the worst case scenario if we should have a patient with this disease and there is any suggestion of person-to-person transmission." She said these would be similar to measures for containing regular flu and SARS.

According to the CDC, outbreaks of H5N1 flu have been reported among poultry in Cambodia, China, Indonesia, Japan, Myanmar, South Korea, Thailand and Vietnam. Another strain of the disease has been seen in wild flocks or poultry in Laos, Pakistan and Taiwan.