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Another early suspect was ornithosis, a disease transmitted through bird droppings and direct handling of infected birds. "The picture in Philadelphia fits ornithosis like a glove," said Dr. Pascal Imperato, chief epidemiologist of the New York City health department. "The symptoms, the fact that this is obviously a common-source outbreak, the fact that there has been no secondary spread of the disease. All these point to ornithosis above all else, and ornithosis is very hard to isolate." An Allentown, Pa., physician, Dr. Gary Lattimer, tended to agree. Assuming that the disease fitted this diagnosis, he treated three of his patients with the antibiotic tetracycline, which combats a number of illnesses, and reported that all were improving.
This hypothesis, however, also failed to test out. CDC researchers screened the tissues for evidence of antibodies to bird-carried viruses. The results were negative. CDC tests found no indication of either plague or typhoid fever. So the search went on into more exotic terrain. Tests also ruled out tularemia (rabbit fever), a deadly tropical disease known as Lassa fever, and Marburg disease, a viral disease from Africa. Further screening seemed to dismiss fungi as a suspect; no fungus is known to produce the fatally fulminating pneumonia typical of Legion disease.
Each possibility dismissed narrowed the track. By week's end Dr. Leonard Bachman, Pennsylvania's secretary of health, suspected that some unidentified natural toxin could have been responsible for the outbreak, and the CDC tended to agree. Using this suspicion as a hypothesis, epidemiologists are now taking another look at the restaurants in which the Legionnaires ate and the hotels in which they stayed, and are studying environmental conditions to determine if they might have played a role in the disease. They are investigating the possibility that the conventiongoers were exposed to some kind of poisonous substance during their stay.
Whatever killed the Legionnaires, the disease detectives concede, may, in the end, prove impossible to detect. "There's an outside chance we may never find out the cause," said CDC Director David Sencer. "I think we will. But there are times when disease baffles us all. It may be a sporadic, a onetime appearance."
Whatever the solution—or the lack of one—to the mystery of the Philadelphia killer, the outbreak served as a jarring reminder that all the marvels of modern technology have not yet made the U.S. immune to a sudden pestilence. Indeed many medical experts warn that the U.S. is still largely unprepared for an onslaught of swine flu, which some fear could hit the U.S. this winter. The new flu, although probably not as dangerous as the World War I variety, could be at least as virulent as the Asian strains that have swept the country in recent years.