TWO YEARS ago a mysterious ailment struck 221 persons in Philadelphia, most of whom were attending an American Legion convention. Thirty-four of the victims died. Because scientists were unable to explain anything at all about the malady, which all too glibly came to be known as "Legionnaires' Disease" or "Legion Fever," the outbreak provoked wild speculation about a new, upstoppable, deadly virus. Last month an outbreak of the same sickness was discoverd among workers in New York City's garment district. So far, there have been six confirmed cases, two deaths and 97 suspected cases. This time, however, there has been little of the fear and hysteria that followed the Philadelphia outbreak - and that illuminates the welcome, dramatic advance of scientific knowledge about "Legion Fever."

In fact, scientists know a great deal about the sickness. They know that it is not a new disease, but one of several pneumonia-like illnesses. They also know that only some of those who are infected become seriously ill, that about one in six of those who do will die, and that the illness's symptoms include abdominal and chest pain, lung congestion, muscle aches, vomiting and a rapidly rising fever. They know that the illness isn't contagious and occurs both sporadically and in outbreaks (there have been 366 sporadic cases and two small outbreaks since 1976). And finally, they know that erythromycin, a commonly used antibiotic, is an effective treatment for the illness.

That is an extraordinary amount of information to have gathered in just two years. What it tells us is not that we are threatened suddenly by a new disease but that we have developed a new capacity to diagnose and treat an old disease that has been around without having been precisely identified.

Scientists still don't know exactly how the bacterium that causes the illness actually infects people or how to diagnose it quickly. Nor do they know where it comes from. One prominent theory holds that the bacterium thrives in soil; another is that it thrives in stagnant water. The information that is known, however, has enabled New York City and federal health officials to act quickly and deliberately in response to the presence of the illness. City officials have kept the public informed about the illness and the progress of their investigation. As a precautionary measure, they have drained water tanks on all buildings and daily wash down streets and subway stations in the affected area. And, along with federal officials, they are beginning exhaustive studies of workers and working conditions there in an effort to trace the origins of the illness and determine why some individuals get it and others do not. Some of those actions may later be found to have been unnecessary. For the moment, however, they are an essential part of the ongoing scientific detective work that, in a short time, has come so close to solving the mystery of "Legion Fever."