The top U.S. health official toured a Vietnamese market Saturday where live chickens are sold to get a firsthand look at the difficulties of controlling bird flu.

Health and Human Services Secretary Mike Leavitt (search) was finishing up a four-country tour of Southeast Asia a week after officials from 80 countries met in Washington to discuss how to prevent and contain a potential global health threat.

After watching chickens and ducks being gutted and cleaned on the sidewalk, Leavitt said it would be difficult to change behavior that is a regular part of people's lives.

"It's evident to me that part of the dilemma here is the cultural momentum because it's happened for hundreds of years, and the chances of changing it anytime soon are very low," he said while walking through the open-air market. "It adds obviously to the equation and to the possibility that you will see an outbreak at some point."

During Leavitt's Southeast Asian trip, which also included stops in Thailand, Cambodia and Laos, he has repeatedly warned of a potential flu pandemic and the importance of all countries rallying together to quickly come up with preparedness plans. The deadly H5N1 (search) bird flu virus was confirmed in birds in Turkey this week.

The disease has hit Vietnam harder than any other country, killing 43 people and 45 million birds, decimating the flocks of poor farmers.

Health experts have warned that the world is due for the next pandemic and fear that the H5N1 virus could mutate into a form that is easily passed among people.

Leavitt on Saturday compared the spread of a pandemic to a brush fire, and said the "chances are not good" of being able to monitor the "spark" when a virus mutates and then quickly contain it.

"There is a spark where every fire starts, and if you're able to be there at the moment it occurs it's possible to simply stamp it out," he said. "If you allow it to burn for an hour, often it will gain enough momentum that it's uncontainable and begins to start other fires."

The U.S. has committed $25 million to address the threat in Asia, and Vietnam will receive more than $6 million for preparedness.

Leavitt was traveling with U.S. Centers for Disease Control and Prevention (search) Director Dr. Julie Gerberding, U.S. National Institute of Allergy and Infectious Diseases (search) Director Dr. Anthony S. Fauci, and Margaret Chan, the World Health Organization's (search) top official in charge of monitoring avian influenza.

Fauci said the potential severity of a bird flu pandemic remains "extraordinarily variable."

"We as public health officials ... must assume the worst-case scenario, and H5N1 now is giving us a lot of signs that it is becoming a little bit more worrisome, if not a lot more worrisome, because of the events that are going on," he said.

Two of the last three global pandemics, all in the 20th century, originated in Asia.

The Asian flu of 1957-58 and the Hong Kong flu of 1968-69 each killed more than 1 million people. Neither compared to the Spanish flu of 1918-19, which killed up to 40 million people and sickened an estimated 20 percent to 40 percent of the world's population.

So far, most human cases have been linked to contact with birds. More than 60 people have died from the virus in Southeast Asia since the disease began ravaging poultry stocks in the region two years ago.