HONG KONG (Reuters) - A Chinese virologist who helped fight SARS and bird flu warned on Monday of a possible swine flu pandemic that the most populous countries in Asia, China and India, would be ill-prepared to handle.

“We are counting down to a pandemic,” said Guan Yi, a professor at the University of Hong Kong who helped trace the outbreak of SARS in 2003 to the civet cat.

“I think the spread of this virus in humans cannot possibly be contained within a short time ... there are already cases in almost every region. The picture is changing every moment.”

Guan, who has been studying and tracking the spread of the H5N1 bird flu virus ever since it was discovered in people in Hong Kong in 1997, said there would be “many problems” if swine flu reached China and India, “where populations are so dense and health infrastructure is still insufficient.”

The virus, which carries swine, avian and human DNA and the designation H1N1, has already killed up to 103 people in Mexico, infected 20 in the United States and six in Canada.

There are many questions surrounding this virus, such as why it appears milder in the United States and deadlier in Mexico.

“It may seem weaker for now in the United States, but we do not know if it will get more virulent when it goes to another place as it mutates constantly,” said Guan.

“When it goes into a place like China, there will be very high transmissibility among people.”

Microbes like viruses mutate all the time and can swap or mix DNA with other viruses they come into contact with. And nobody knows whether they could become more or less deadly, experts say.

Guan said the swine flu virus was very different from the seasonal human H1N1 flu virus.

“It is almost a new subtype,” he said, adding that as it was already transmitting efficiently among people, the world already had a pandemic on its hands.

Currently the World Health Organisation classifies the virus as a “public health emergency of international concern” that could become a pandemic, or global outbreak of serious disease.

“This is what I am very worried about. The WHO is always very cautious (about raising its alert system) but it is wasting time,” Guan added.

The current phase of alert is 3 on a scale of 1 to 6. A full-blown pandemic, level 6, denotes sustained human to human spread over many countries of a new and serious virus.