The most dangerous influenza pandemics arise when human and bird flu viruses combine in pigs to form completely new viral types according to a report in the current issue of the Journal of Virology published by the American Society for Microbiology. The study by researchers in Japan, Brazil, Germany, the US and Italy involved complex genetic studies of the viruses that caused two major pandemics, Hong Kong in 1968 and Asian in 1957. They also studied cells in the throats of pigs which contain receptors for both human and bird flu types. It has long been thought that pigs served as "mixing bowls" where viruses could replicate and reassort themselves into new forms. "The evidence we present supports the role of pigs as a source of potentially hazardous influenza viruses," the authors conclude, adding that swine populations should regularly be monitored for avian-like influenza viruses.