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WASHINGTON (Reuters) - The H5N1 bird flu virus, which has now sickened more than 400 people globally, is infecting birds and people all across China and is still a cause for serious concern, flu experts said on Tuesday.
Recent cases are the expected winter seasonal surge seen for many types of viruses, they said, and there is no evidence that it is mutating into a more dangerous form.
"In the past three months, we have continued to see a great deal of activity," Dr. Keiji Fukuda of the World Health Organization told reporters.
Chinese officials confirmed a new case on Monday in a 23-year-old woman in Hunan province, who was being treated.
All the patients were around chickens, so there is little concern that people are passing the virus to one anther, the flu experts told a news briefing at a meeting in Washington of the Infectious Diseases Society of America.
"These cases and the widespread nature of them continue to point out how persistent this virus is and how widespread it is," Fukuda said. "We believe that the threat of pandemic influenza remains as high as ever."
Since 2003, the H5N1 avian influenza virus has infected 404 people in 15 countries and killed 254 of them. It has killed or forced the destruction of more than 300 million birds as it spread to 61 countries in Asia, the Middle East, Europe and Africa.
"This is a very unusual virus and the reason why it so unusual is that it is probably the most lethal influenza virus that has ever been discovered," Fukuda said.
"It really has an amazing ability to kill a variety of birds and mammals, including people."
Fukuda said China had investigated each case thoroughly, and he did not see any especially worrying signs in the eight recent cases.
"It has been noticed that the people who have been infected are spread across a wide area of China. What that tells us is that the virus is widespread in China. But we have known for a long time that it is widespread in Southeast Asia in general," he said.
It is also known that vaccinating birds against H5N1 is not 100 percent effective, so it is not surprising that individual birds would become infected and spread the virus to people, he said.
While H5N1 rarely infects people, experts fear it could mutate into a form that people could easily pass to one another, sparking a pandemic that could kill tens of millions and topple the global economy.
Dr. Nancy Cox of the U.S. Centers for Disease Control and Prevention said while the virus is mutating steadily, it is not taking on any startling changes yet.
"We have not seen changes in the genes of H5N1 viruses that would be indicative of the kind of change that we would expect were the virus to become more transmissible from human to human," she said.
Fukuda and other experts praised China for being so quick to report and investigate each case.
"Hopefully, we will see this become the standard behavior for most countries," Fukuda said.
Experts say the odds of an influenza pandemic of some sort are 100 percent. No one can say when or where it might happen, or what strain of flu might cause it, and Fukuda noted that H5N1 is not the only threat.