The emergence of SARS and other novel human diseases in Asia is frequently attributed to the proximity of humans to their livestock. But, says Corrie Brown, professor at the University of Georgia College of Veterinary Medicine in Athens, human-livestock cohabitation occurs in many places that are not noted for new disease, such as Latin America. "It's a dogma we've accepted without proof," she said.

More human diseases will come from our pets, predicted Brown. "Maybe SARS is an example of that," she speculated. Perhaps a cat acted as initial mixing vessel for combining noninfective existing viruses into something that could make people very sick and even kill them, she says

There is much speculation that SARS jumped to people from domestic poultry, but no molecular evidence backs it up, agreed Jacqueline Katz, chief of the immunology and viral pathology section of the influenza branch at the Centers for Disease Control and Protection (CDC).

However, as microbiologists grapple with the question of why emerging diseases so often do their emerging in Asia, Katz said that the continent's enormous population, its crowding, and the common practice of living near livestock, especially poultry are likely to be part of the answer. Live bird markets are perfect for transmitting viruses from poultry to humans, she says.

Whatever the origin of SARS, there is little doubt that domestic poultry viruses can cause fatal human respiratory disease. The 1997 Hong Kong outbreak of avian flu in humans was stopped in its tracks by slaughtering millions of chickens in Hong Kong markets, said Katz.

A different kind of avian flu sickened two people in Hong Kong in February and killed one of them. Its origin is not clear. The affected family might have caught it from chickens on a New Year visit to the mainland or perhaps from wild aquatic birds - infected birds have been found in Hong Kong parks.

Owing to the dominance of SARS in the news, two other new avian flu strains have been wreaking havoc in the past few weeks without much media attention. One has turned up on turkey farms in Connecticut and is creating problems for the US Department of Agriculture, says Katz.

The other is a virulent strain of poultry flu, which appeared in the Netherlands at the end of February. It soon jumped directly from Dutch birds to Dutch pigs, and has also been conveyed to humans, probably by an infected poultry worker, she said.

In humans, this new bird virus has caused conjunctivitis - 82 cases to date. Some patients also suffered respiratory illness, but there's no hard evidence that this symptom was caused by the virus. The disease has now spread to Belgium and Germany, and has cost 100,000 Euros to treat in less than 3 months.

The gaps in so-called species barriers are wide and deep, concludes Brown. Practically all carnivore diseases affect more than one species, she says, as do 60% of livestock diseases. Furthermore, there is still almost nothing known about pathogens in wild animals.

Around 75% of new human diseases are zoonotic, says Brown. Of some 1400 known human pathogens, 60% came from animals. And the traffic is two-way, she says, with humans sharing many of their diseases with animals. Examples include tuberculosis and transmission of Giardia to bears in Yosemite Park. "There is so much out there that we have a lot of job security," Brown told fellow microbiologists.