clunky imprecision of that sentence is why ecologists prefer math.) So the first rule of a successful parasite is slightly more complicated than Don’t kill your host. It’s more complicated even than Don’t burn your bridges until after you’ve crossed them. The first rule of a successful parasite is βN/(α + b + v).
The other thing that makes Anderson and May’s 1982 paper vivid is its discussion of myxoma in Australian rabbits. That brought their modeling to an empirical case and allowed them to test theory against fact. They described Frank Fenner’s five grades of virulence. They saluted his methodical combination of field sampling and lab experiments. They mentioned the mosquitoes and the open sores. Then, using Fenner’s data and their own equation, they plotted a relationship between virulence and success. Their result was a model-generated prediction: Given this rate of transmission, given that rate of recovery, given those unrelated mortalities, then . . . an intermediate grade of virulence should come to predominate.
Son of a gun, it matched what had happened.
The match showed that their model, though still crude and approximate, might help predict and explain the course of other disease outbreaks. “Our major conclusion,” wrote Anderson and May, “is that a ‘well-balanced’ host-parasite association is not necessarily one in which the parasite does little harm to its host.” Their italics: not necessarily. On the contrary, it depends. It depends on the specifics of the linkage between transmission and virulence, they explained. It depends on ecology and evolution.
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Anderson and May were theoreticians who worked much with other people’s data. So is Edward C. Holmes. Unlike them, he’s a specialist in viral evolution, one of the world’s leading experts. He sits in a bare office at the Center for Infectious Disease Dynamics, which is part of Pennsylvania State University, in a town called State College, amid the rolling hills and hardwoods of central Pennsylvania, and discerns patterns of viral change by scrutinizing sequences of genetic code. That is, he looks at long runs of those five letters, A, C, T, G, and U, strung out in unpronounceable streaks as though typed by a manic chimpanzee. Holmes’s office is tidy and comfortable, furnished sparsely with a desk, a table, and several chairs. There are few bookshelves, few books, few files or papers. A thinker’s room. On the desk is a computer with a large monitor. That’s how it all looked when I visited, anyway.
Above the computer hung a poster celebrating “the Virosphere,” meaning the unplumbable totality of viral diversity on Earth. Beside that, another poster showed Homer Simpson as a character in Edward Hopper’s famous painting “Nighthawks.” I’m not sure what that one was celebrating, unless perhaps donuts.
Edward C. Holmes is an Englishman, transplanted to central Pennsylvania from London and Cambridge. His eyes bug out slightly when he discusses a crucial fact or an edgy idea, because good facts and ideas impassion him. His head is round and, where not already bald, shaved austerely. He wears wiry glasses with a thick metal brow, as in old pictures of Yuri Andropov. Though shaved, though brilliant, though Andropovian at first glance, Edward C. Holmes isn’t austere. He’s lively and humorous, a generous soul who loves conversation about what matters: viruses. Everyone calls him Eddie.
“Most emerging pathogens are RNA viruses,” he told me, as we sat beneath the two posters. RNA as opposed to DNA viruses, he meant, or to bacteria, or to any other type of parasite. He didn’t need to cite the particulars about RNA viruses because I already had that list in my mind: Hendra and Nipah, Ebola and Marburg, West Nile, Machupo, Junin, the influenzas, the hantas, dengue and yellow fever, rabies and its cousins, chikungunya, SARS-CoV, and Lassa, not to mention HIV-1 and HIV-2. All of them carry their genomes as RNA. The category does seem to encompass much more than its share of dastardly zoonoses, including most of the newest and the worst. Some scientists have begun asking why. To say Eddie Holmes wrote the book on this subject wouldn’t be metaphorical. It’s titled The Evolution and Emergence of RNA Viruses, published by Oxford in 2009, and that’s what had brought me to his door. Now he was summarizing some of the highlights.
Granted, Eddie said, there are an awful lot of RNA viruses generally, which might seem to raise the odds that many would come after humans. RNA viruses in the oceans, in the soil, in the forests, and in the cities; RNA viruses infecting bacteria, fungi, plants,