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What those might be is also anybody's guess. "There have been cases," says Meselson, "where a man works in a factory with anthrax spores and doesn't get sick, but comes home, takes off his clothes, and his wife gets inhalation anthrax. There are also cases where a person waiting for a bus some distance away from a factory where spores are known to exist gets inhalation anthrax, although not all workers in the factory do." It may have to do with how deeply an individual breathes in the spores or with his or her overall respiratory health. It might even be related to age. So far, the vast majority of fatal inhalational-anthrax cases, both in the past few weeks and in the Soviet accident, occurred in victims who were at least 40 years old.
Another confounding factor has to do with the behavior of anthrax particles. Ken Alibek, who ran the Soviet and Russian biological-weapons program until 1992 and later defected to the U.S., says that aerosolized anthrax can travel in unpredictable ways. The weapons-grade powder he worked with, he recalls, kept turning up in odd places in the labs. This also seems to be happening at the Brentwood mail facility outside Washington, which processed the Daschle letter. The CDC's contamination map of the building reveals several different locations where anthrax was found, in no discernible pattern.
Yet another question is the incubation period of the spores. Evidence from the Sverdlovsk accident indicates that victims can develop symptoms as long as 45 days after exposure--suggesting that more victims could still show up in the U.S.
If anthrax really can move around erratically and cause disease at very low concentrations, Kathy Nguyen's death becomes far easier to explain. Her mail might have passed through the Morgan postal station in New York City, for example, where letters to NBC and the New York Post were handled, or she might have had some sort of contact with someone who worked there. On the other hand, some FBI agents are convinced that their best lead in the case is lurking somewhere in the last weeks of Nguyen's life. "This is the hot one," says an agent. "If we can figure out what's staring us in the face, it'll break it. It's got to be an apartment near her, or somebody where she worked or someplace she went."
Unfortunately, it won't be easy to reconstruct Nguyen's movements over the past few weeks: by the time her disease was diagnosed she was on a respirator and heavily sedated. She died before she could be questioned.
Meanwhile, scientists are making the best use of the data pouring in. "This is new ground," says Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "Years from now, people will look at our experience and say, 'Ah, we know spores can do A, B, C, D and E, but in October 2001, they weren't sure of that.'" Like many experts, Fauci is willing to consider that anthrax may be far easier to catch than anyone thought--but like his counterparts on the criminal side of the investigation, he's also open to the idea that terrorists have been releasing spores in other ways.