while preventing immune cells or bac- teria from entering. The placenta is es- sentiallya modified egg. In the early nineteen-seventies, biologists who were scanning baboon placentas with an elec- tron microscope were surprised to see retroviruses on a layer of tissue known as the syncytium, which forms the princi- pal barrier between mother and fetus. They were even more surprised to see that all the animals were healthy. The same phenomenon was soon observed in mice, cats, guinea pigs, and humans. For many years, however, embryologists were not quite sure what to make of these placental discoveries. Most re- mained focussed on the potential harm a retrovirus could cause, rather than on any possible benefit. Cell fusion is a funda- mental characteristic of the mammalian placenta but also, it turns out, of endog- enous retroviruses. In fact, the protein syncytin, which causes placental cells to fuse together, employs the exact mecha- nism that enables retroviruses to latch on to the cells they infect. T he Nobel Prize-winning biologist Joshua Lederberg once wrote that the "single biggest threat to man's contin- ued dominance on this planet is the virus." Harmit Malik, an evolutionary geneticist at the Fred Hutchinson Cancer Research Center, acknowledges the threat, yet he is confident that viruses may also provide one of our greatest scientific opportuni- ties. Exploring that fundamental para- dox-that our most talented parasites may also make us stronger-has become Malik's passion. "We have been in an evolutionary arms race with viruses for at least one hundred million years," he told me recently, when I visited his laboratory. "There is genetic conflict everywhere. You see it in processes that you would never suspect; in cell division, for in- stance, and in the production of proteins involved in the very essence of maintain- ing life. "One party is winning and the other losing all the time," Malik went on. "That's evolution. It's the world's defini- tive game of cat and mouse. Viruses evolve, the host adapts, proteins change, viruses evade them. It never ends." The AIDS virus, for example, has one gene, called "va:" that does nothing but block a protein whose sole job is to stop the virus from making copies of itself. It simply takes that protein into the cellu- lar equivalent of a trash can; if not for that gene, H.I. V. might have been a triv- ial disease. "To even think about the many million-year processes that caused that sort of evolution," Malik said, shak- ing his head in wonder. "It's dazzling." Malik grew up in Bombay and studied chemical engineering at the Indian Insti- tute of Technology there, one of the most prestigious technical institutions in a country obsessed with producing engi- neers. He gave no real thought to biol- ogy, but he was wholly uninspired by his other studies. "It was fair to say I had lit- tle interest in chemical engineering, and I happened to tell that to my faculty ad- viser," he recalled. "He asked me what I liked. Well, I was reading Richard Dawkins at the time, his book 'The Selfish Gene' "-which asserts that a gene will operate in its own interest even if that means destroying an organism that it inhabits or helped create. The concept fascinated Malik. "I was think- ing of becoming a philosopher," he said. "I thought I would study selfishness." Malik's adviser had another idea. The university had just established a depart- ment of molecular biology, and Malik was dispatched to speak with its director. "This guy ended up teaching me by him- self, sitting across the table. We met three times a week. I soon realized that he was testing out his course on me. I liked it and decided to apply to graduate school-although I had less than a tenth of the required biology courses. I had very little hope." But he had excellent test scores and in 1993 was accepted at the University of Rochester, as a graduate student in the biology department. He visited his new adviser as soon as he ar- rived. "He looked at my schedule and said, 'I see that you are doing genetics.' I had no clue what he was talking about, but I said sure, that sounds good. I had never taken a course in the subject. He gave me the textbook and told me that the class was for undergraduates, which made me feel more comfortable." It wasn't until the end of the conversation that Malik realized he would be teaching the class, not taking it. The Hutchinson Center encourages its research scientists to collaborate with colleagues in seemingly unrelated fields. Malik and Michael Emerman, a virolo- gist at the center's Human Biology and Basic Sciences Divisions, have been working together for four years. Malik's principal interest is historical: why did evolutionary pressures shape our de- fenses against viruses, and how have they done it? Emerman studies the genetic composition and molecular pathology of the AIDS virus. "Together, we are try- ing to understand what constellation of / / 3fbC "These maps are old, so pay no attention to the borders. "