So far, it does not appear that the disease can be easily transmitted from human to human, but experts say a bird flu virus mutation is quite possible. Chan also said she believes the available evidence indicates that the world is due for a pandemic soon.

"We owe it to the community we are serving to keep watch, to be vigilant, on the lookout for early signs of a pandemic. The risk is there," she said. "We should not let our guard down. We are very concerned about the likelihood of a pandemic and particularly its impact on countries with weak health systems and in countries in Africa, where they have many people already affected by HIV/AIDS and other diseases."

A hard fight

Three factors make the bird flu battle more difficult. The first is that the disease can be spread by migrating wild birds, which are difficult to control and vaccinate, and perhaps by other species.

In mid-February, Japanese scientists feared that rats were responsible for carrying avian flu to four poultry farms in south Japan in January, which would be an alarming development that suggests the virus could spread more quickly than realized.

Toshihiro Ito, a professor of veterinary microbiology at Tottori University, who heads the team of specialists investigating the situation, told the Asahi Shimbun, "It's possible that small rodents, such as rats, carried the virus into the chicken coops." If this were confirmed, it would be almost impossible for farmers to avoid similar outbreaks.

The second issue is that currently there is no "silver bullet" to kill the virus. Experts say that a new medication, bacterin, will not be available on the market for the next one to two years. Although another drug, Tamiflu, is considered the most effective medicine, it is an oral treatment that does not prevent the flu. Instead, it lessens the severity of the symptoms and shortens recovery time by one to two days.

Tamiflu works best if it is taken within 48 hours after the first symptoms of flu appear. In addition, the U.S. Food and Drug Administration (FDA) issued a warning in November, noting reports of self-injury and delirium in flu patients taking Tamiflu, mainly among children. The FDA had received reports of 103 cases by then--95 from Japan, five from the United States and three from other countries. There was no direct evidence proving conclusively that Tamiflu causes those side effects, but the drug's manufacturer, Roche, and the FDA are continuing to monitor the situation.

The third factor is that evidence shows that global warming is worsening the bird flu situation. An international research group recently found that many diseases, including bird flu, are spreading more rapidly due to global warming.