The recent swine flu outbreak has caused worldwide anxiety. But there's one thing we don't need to be anxious about: We are not facing a pandemic. As flu virologists have always defined them, pandemics involve totally new viruses to which no one has any immunity, allowing them to spread rapidly and destructively.
Over the last 90 years, there have been, as far as we know, three totally new such viruses: the infamous "Spanish flu" of 1918; and two arising out of southern China, one in 1957, one in 1968. All of these involved shifts or replacements in the H (hemagluttinin) surface protein; the 1957 strain involved a change in the N (neuraminidase) surface protein as well.
But this swine flu is a new form of an old nemesis, the H1N1 virus, which caused the outbreak of 1918 and which has been circulating widely since the "Russian flu" outbreak reintroduced H1N1 to the human population in 1977. No one has undertaken lab studies yet to show how much immunity prior infections will provide, but according to the gray eminence of flu research, Edwin K. Kilbourne, who has treated flu patients since 1947, one reason this flu seems more virulent in young people is that older people's immune systems remember H1N1 very well; they've been infected by it in the past. And happily, at least so far, the vast majority of young people seem quite capable of fighting it off.
As with any new outbreak, unraveling all of this flu's mysteries will take time. But, 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.
