Health experts are in China in a bid to trace the source of the flu bug that has killed six people in Hong Kong.

The Hong Kong bird flu virus has sparked fears of a pandemic as scientists worry that the disease, previously only found in chickens, may have mutated into a form that could pass between humans.

Scientists are also working on finding a vaccine

World Health Organisation scientists suspect the bug originated in southern China - a theory vigourously denied by the Chinese authorities.

The 14-man team from the WHO and the Centres for Disease Control, in Atlanta, Georgia, is in Guangdong province, southern China, visiting poultry farms and looking for traces of the virus.

As the team began their work a new study was published showing similarities between Hong Kong bird flu and Spanish flu, which killed between 20 and 40 million in three years from 1918.

More people died from Spanish flu - the world's worst influenza outbreak - than were killed in World War I.

In Hong Kong six people have died out of the 18 known cases of infection - a 33% fatality rate.

Scientists from Hong Kong and the CDC say the new bird flu, H5N1 influenza A, bears the hallmark of highly dangerous viruses.

Nurses donned face masks to treat Spanish flu victims

Writing in the journal Science, they point to an arrangement of the amino acid protein building blocks which was characteristic of "highly pathogenic avian influenza A viruses". The same structure was seen in the 1918 Spanish Flu virus.

Led by Kanta Subbarao, head of CDC's molecular genetics laboratory, they said it had been suggested that the acquisition of bird flu genes by the human influenza A virus could produce a lethal cocktail.

A man covers his face for protection in Hong Kong

Their paper said: "A pandemic of influenza could begin with isolated cases, in which avian or swine influenza viruses adapt to human hosts, or, over time, genetically re-assort with circulating human influenza A viruses.

"Alternatively it could begin as a rapid and explosive spread of a pandemic virus derived from a re-assortment event in an intermediate host."

Because the virus is completely new people have little natural immunity to it.

New flu strains can develop when viruses reshuffle their genetic codes while passing from one species to another.

Most flu viruses that are highly dangerous in birds do not replicate efficiently in humans.

But occasionally the genetic mutation produces a virus against which humans have little immunity, bringing with it the threat of a pandemic.

Scientists believe most flu viruses start out in birds, but bird flu is very different from human flu and normally cannot infect people directly.

Pigs can be infected by both bird and human flu viruses and often act as a mixing bowl for influenza genes.

The CDC team is now checking viruses taken from all the human H5N1 victims to see if they all have the same qualities.
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