Profile: A Host with Infectious Ideas; May 2001; Scientific American Magazine; by Steve Mirsky; 2 Page(s)

AMHERST, MASS.-Newton had a falling apple. Darwin mused on finches. Paul W. Ewald's inspiration was diarrhea. "I wish I had something more romantic," says the Amherst College evolutionary biologist. It gets uglier: Ewald, then a graduate student studying bird behavior, was camped near a Kansas garbage dump. As he waged a three-day battle against his sea of troubles, he contemplated the interactions between a host-himself, in this case-and a pathogen. "There's some organism in there," Ewald remembers thinking during that 1977 experience, "and this diarrhea might be my way of getting rid of the organism-or it might be the organism's way of manipulating my body" to maximize its chances of passage to the next victim by, for example, contaminating the water supply. "If it's a manipulation and you treat it, you're avoiding damage," he notes. "But if it's a defense and you treat it, you sabotage the host."

Host-pathogen relationships have dominated Ewald's thoughts ever since, leading to numerous articles, two books and, depending on whom you talk to, the respect or scorn of scientists and physicians. The admiration comes from those who think he was on to something really big in his earlier publications, which he summed up in his 1994 book Evolution of Infectious Disease. "I think that Paul Ewald has been a pioneer in using evolutionary theory to attack hard questions in pathogenesis," comments Stephen Morse, a virologist and epidemiologist at Columbia University. "His work has, for the first time, shown a way to generate testable hypotheses to study such questions as the evolution of virulence-once thought intractable-and infectious causes of chronic diseases." Indeed, the Atlantic Monthly referred to Ewald as "the Darwin of the microworld" (to which Ewald responds, "No, Darwin is Darwin of the microworld, too").