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.
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. Locals report a fearful stench, hosts of flies and, since December 2008, serious respiratory disease that sickened 60% of the community. One of those cases, a 5-year-old boy who has since recovered, had the H1N1 swine flu virus. Other samples have disappeared, Kuyek says, and most people were never tested.
Influenzas that have their origins in huge, crowded animal farms are often more virulent than other flu strains. Germs that kill their hosts quickly tend not to thrive; their hosts die before there is time to pass the virus on. But on crowded farms, the next snout is an inch away, and even virulent strains can gain a foothold. It is the same type of conditions that produced deadly avian influenza in giant poultry farms in Asia over the last 10 years.
"This is human hubris," says Earl Brown, flu virologist from the University of Ottawa. "People tend to think, 'We'll manage this thing.' Chicken farming is way ahead of pig farming in dealing with serious infections, but pig farms are catching up. The bird flu was not able to adapt to humans, but swine are much closer to us than birds."
None of this is to say we shouldn't worry. Even the milder flu variants now spreading around the world will pose serious public health issues. Host factors -- pregnancy, cardiac conditions, weakened immunity -- may mean that even milder strains can kill.
And the flu won't be easily stopped. Flu virologist Richard Webby of St. Jude Children's Research Hospital in Memphis notes that although quarantine and isolation, strictly applied, could stop the spread, implementing those things worldwide is virtually impossible.
And we can predict the virus will only spread more easily over time: Natural selection ensures, by definition, that the most transmissible strains will be passed on. Fully adapted human flu is explosive -- no quarantine, no isolation, no Tamiflu can contain it.
Natural selection theory also tells us that whatever we will face, it won't be another 1918. As Ewald has argued for years, only packed conditions allowing deathly sick hosts to pass disease repeatedly to the well can produce highly virulent strains of flu -- for animals or for people. The usual sort of human crowding will not do it. Even massive, densely populated Mexico City, with more than 20 million inhabitants, won't produce the kind of lethal strains that the Western Front did in World War I. People died in Mexico because they were close to the epicenter of the disease, to the probable emergence of lethal strains from crowded pig breeding. But natural selection's corrective action is swift and predictable: The strains spreading across the world are milder.
Nevertheless, this new flu is with us, perhaps to stay. This is not good. But it is not a world-ending catastrophe either.