All Intensive Purposes

Writing in the LA Times (registration required), Wendy Orent disputes claims that avian flu could cause a deadlier pandemic than that in 1918:

Infectious disease expert Michael Osterholm insists that because the planet is far more crowded and transportation more rapid than in 1918, H5N1 could cause the worst disease outbreak in history.

But this logic has more holes than a tennis racket. First, avian flu evolved to kill chickens, not people. It attacks people who drink infected chicken blood, wade in chicken feces or slaughter sick chickens for food. In other words, its human victims have typically been exposed to massive doses of chicken virus.

Second, H5N1 has evolved great virulence among chickens only because of the conditions under which the animals are kept — crammed together in cages, packed into giant warehouses. H5N1 was originally a mild virus found in migrating ducks; if it killed its host immediately, it too would die. But when its next host's beak is just an inch away, the virus can evolve to kill quickly and still survive.

As evolutionary biologist Paul Ewald of the University of Louisville has pointed out, the same process "cooked" the virulence of the 1918 flu, though instead of chickens in cages it was soldiers in World War I's trenches, hospitals and transports who fell victim to a virus that became increasingly deadly as it cycled among them.

It is faulty logic to expect that, in the absence of trench warfare or other human "disease factories," flu will evolve to be a pandemic terror. And no one has identified such conditions near the chicken farms of Southeast Asia. In other words, if avian flu ever does adapt enough to spread easily to humans, its lethality will have to drop. It may well cause another pandemic, as we indeed have no resistance to it, but it cannot be a pandemic as lethal as the 1918 flu.

Which is not to say that it won't kill lots of people -- just a different magnitude of "lots." As a public health matter, Orent suggests we avoid massive trench warfare and other conditions ripe for a flu to spread. Hard to argue with that, though it might be easier to scare up funding by drawing the parallels to 1918.