Mexican swine flu not so scary

using the lens of Darwinian evolution, certain aspects are starting to come into focus. For one thing, it's clear that the virus, which originated in Mexico, is most virulent in that country. The 1,000 or so reported Mexican cases have been either fatal or severe enough to require hospitalization. But because of natural selection, the strains spreading across the world are milder.

According to evolutionary biologist Paul W. Ewald of the University of Louisville, human influenza is usually a mild to moderate disease because it depends on host mobility to spread. The U.S., Canadian and New Zealand teenagers on their spring breaks did not sit in hospitals with the very sick and dying; they mingled with people who were sneezing and coughing but walking around, riding subways, perhaps going to the beach or dancing in nightclubs. People don't start being really infectious until they show symptoms, and whatever symptoms those people had must have been mild enough to remain out in public. The strains sent out around the world were, by definition and necessity, milder than the most lethal strains.

So why are some of the Mexico strains so lethal? The answer may lie in the virus' possible origin: a giant Veracruz pig farm that raises almost a million pigs a year. According to Devlin Kuyek of GRAIN, an environmental organization, reports have been coming in for months of the appalling conditions in the Perote Valley where the farm is located.

there is more evidence that a serious pandemic is not imminent. In 1976 there was an outbreak of an H1N1 swine virus in Fort Dix, N.J., which showed human-to-human transmission but did not go on to become a highly virulent strain. This virus was very similar to regular swine influenza viruses and did not show a high affinity for the human host.

Although the swine virus currently circulating in humans is different from the 1976 virus, it is most likely not more virulent than the other seasonal strains we have experienced over the last several years. It lacks an important molecular signature (the protein PB1-F2) which was present in the 1918 virus and in the highly lethal H5N1 chicken viruses. If this virulence marker is necessary for an influenza virus to become highly pathogenic in humans or in chickens -- and some research suggests this is the case -- then the current swine virus, like the 1976 virus, doesn't have what it takes to become a major killer.