A flu pandemic can come about in two ways. One way is for the virus to randomly mutate to become easily transmissible between humans. "Randomly" is the key word here. There are no evolutionary pressures to make H5N1 adapt better to humans. Given enough time, H5N1 might mutate so that it could under the right conditions become pandemic. But that could take millions of years, during which time it would be more likely to mutate itself out of existence. H5N1 was first identified in Scottish chickens in 1959. It has been flying around the globe for close to half a century and hasn't done a number on us yet. There's absolutely no reason to think it will pick this year or next to do so.

Another scenario is that somebody with human flu could contract avian flu at the same time and the two flus could "reassort" into hybrid avian-human flu. The last two flu epidemics in the 20th century - 1957-58 and 1968-69 - were caused by such hybrids. We can help reduce this possibility by vaccinating as many people as possible (especially Southeast Asian poultry farmers) against human flu, thus reducing the potential number of "mixing vessels." Programs underway to keep farmers away from poultry droppings and spittle (birds don't sneeze or cough) will also help.

A fascinating study in the August 8, 2006, issue of Proceedings of the National Academy of Sciences would seem to indicate we're already pretty safe from a human-avian hybrid. Researchers from the U.S. Centers for Disease Control and Prevention conducted three separate studies with ferrets, which are among the few animals known to suffer from and transmit human flu. The ferrets were infected with several H5N1 strains in addition to a common human influenza virus (H3N2) that circulates almost every year. The infected animals were then either placed in the same cage with uninfected ferrets to test transmissibility by close contact or in adjacent cages with perforated walls to test spread of the virus from respiratory droplets.

The research showed that the H3N2 virus passed easily by droplets (ferrets do sneeze and do not use handkerchiefs) but the H5N1 virus did not spread - the same thing we're seeing in humans infected with H5N1 from birds.

Separately, the scientists used gene splicing to create a hybrid H5N1/H3N2 virus. In other words, rather than letting nature take its course and seeing if the viruses would reassort, they guaranteed that reassortment occurred. They found these hybrids also did not pass easily between the animals. Moreover, ferrets injected with the reassorted virus showed symptoms less severe than those with the pure avian flu. Reassortment appears to have weakened the virus.