Ducks can pass avian flu viruses to pigs, where two viruses can mingle and form a new strain that is passed on to the farmers. The pigs, which have a genetic make-up similar to humans, act as the "mixing vessels."
Eurasian avian influenza viruses have been recognized as the precursors to the influenza virus genes that re-assorted with human strains to generate the 1957 and 1968 flu pandemics.
However, Scholtissek, who linked China's agricultural practices with those influenza pandemics, said it's unlikely the same thing happened with SARS. While the flu virus' genome is divided into eight separate strands, which can break up and easily mix and match individual pieces to come up with brand new variants, the SARS virus consists of a solid genome.
But very little can be said with certainty about the SARS virus, partly because it's believed to be mutating all the time. Hong Kong scientists this weekend said the SARS virus is mutating rapidly into at least two forms.
"This rapid evolution is like that of a murderer who's trying to change his fingerprints or even his appearance to escape detection," said Dennis Lo, a chemical pathologist at the Chinese University of Hong Kong.
There is little doubt that crowded conditions, like those in China, play a major part in the spread of SARS. Shoppers in Guangdong province, which has a population of 76 million, throng the busy markets for fresh meat. The need to feed a growing population has greatly increased the number of animals raised.
"Viruses transfer when you have a density of population of people and animals," said Ian Jones, a professor at the School of Animal and Microbial Sciences at the University of Reading in England. "Given that about a quarter of the world's population is living in China, things are just bound to pop up there."
As people expand into ever-wilder areas, humans are coming in closer contact with wild animals. In southern China, wild and exotic animals like snakes and birds are treated as delicacies with many people believing they are good for one's health.
Michael Lai, a professor at the University of Southern California in Los Angeles and an expert on coronaviruses, believes that SARS existed in a wild animal, most likely a wild bird, before transferring to humans.
"The SARS viral sequence is different from any known human or animal," said Lai. "The sequence and genome structure of the SARS virus most closely resemble those of an avian coronavirus, [though] it also has a hint of a murine [or rodent] coronavirus."
Add to the mix the Chinese government's capricious handling of the SARS outbreak. Although doctors in southern China were aware of the epidemic as early as January, information about the disease was kept secret by the government for months. It only began to emerge at the beginning of April, under intense international pressure.
For months, doctors in places as far-flung as Canada scratched their heads as new cases of a mysterious pneumonia-like disease arrived at their hospitals. Health officials now charge that the epidemic could have been contained sooner if the Chinese government had been more forthcoming with information about the epidemic.
In a rare public disclosure of failure, Chinese officials late last month admitted that infection rates were many times higher than initially reported, and the government fired both the health minister and the mayor of Beijing.
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