A new outbreak of legion fever gripped a small city in Tennessee yesterday, meaning this mysterious illness has shown up in 19 states, causing at least 54 more cases and 14 deaths since it hit 181 American Legionnaires in Philadelphia 14 months ago.

The case figure will probably go up higher since only three confirmed in the laboratories of the federal Center for Disease Control at Atlanta were counted in the new Tennessee total.

There have actually been 24 cases in all, with four details of "Legion-like" pneumonia at Holston Valley Community Hospital in Kingsport, Tenn., population 31.938, and legion fever has been ruled out so far in only a few.

The Kingsport outbreak comes on top of similar new rashes of the disease in Vermont and Ohio and scattered cases elsewhere. This includes at least two previously unrevealed cases and one death in the District of Columbia.

The number of cases now appearing - and others being identified from previous years - makes it clear to public health officers that legion fever is no freak nor unique event. Instead, it is a kind of pneumonia that had never before been identified as a separate form of that disease.

It is caused by a kind of bacterium that has now been identified. But no one knows yet how the disease is transmitted.

Two confirmed cases have occurred in Washington resident, Dr. Martin Levy, District communicable disease director, said yesterday. A 47-year-old man came down with the illness in June and died in August after lung surgery to try to treat abscesses caused by the disease.

Another man, 55, came down with the disease in August and recovered. A woman from Ohio is now in the hospital here with the illness and "improving," Levy reported.

There may have been another death from the disease here in June, Levy said - in a man in his 80s who had all signs of the legion pneumonia - but health officers couldn't get enough blood samples for a firm laboratory diagnosis.

The cases here were "not something we were rushing to announce, because they seemed to be just a few of a sporadic number of cases around the country," Levy explained.

Since midsummer, however, there have been outbreaks in Columbus, Ohio; then Burlington Vt., and now Tennessee.

Health officers in Tennessee said that though just three legion fever cases have been confirmed, six new patients were hospitalized in the past 24 hours. Legion fever has been ruled out in seven of the previous Kingsport patients.

But federal disease officers are investigating the fact that all those who came down with the form of "a typical pneumonia" that characterizes the disease lived or had visited in the area of the hospital where they are now patients.

The hospital is limiting visitors and has halted work on a building project on the chance that construction dust may play some role in spreading the guilty germs.

Federal officers believe the disease has been spread through the air in some past incidents. They also doubt that the disease is increasing in frequency. What is happening? they think, is that doctors every where are becoming more aware of it - and therefore finding it.