Legionnaire's Disease, which struck down members of the American Legion nearly four years ago in Philadelphia, turns out to be just one of a group of "new pneumonias" being identified by medical scientists.

The ailments are not really new; they have sickened and killed people in the past. But previously doctors were unable to pinpoint their cause, and so were unable to treat them.

Doctors now have an explanation for many of the puzzling pneumonias and often a cure: the same antibiotic, erythromycin, that usually cures Legionnaire's Disease.

Scientists at the federal Center for Disease Control in Atlanta are learning that the "new pneumonias," including Legionnaries's Disease, are caused by similar, and probably a family of bacteria. All are generally rod-like organisms called "gram-negative" for their unusal response to a laboratory staining test.

They sometimes are found in association with water -- in ponds or streams or industrial air-conditioning equipment -- or in newly turned moist soil. They usually are picked up by human beings in the air, though sometimes they are picked up directly by swimmers and skin divers.

All the "newest" bacteria are temporarily being called "LLOs" or Legion-like organisms." But they bear such varied names as Pittsburgh penumonia, WIGA (an abbreviation of a patient's name), Tex-KL (found by Dr. Karen Lewallen in Texas), skin diver's disease and Fort Bragg fever.

There also are some LLOs still in search of a disease. That is, "we haven't found any disease for them yet, though I feel sure some exist," Dr. David Fraser, chief of the disease control center's special pathogens branch, said last week.

"The 'new' pneumonias are commoner than suspected," Dr. Louis Weinstein of Peter Bent Brigham Hospital in Boston wrote in an editorial in the April Annals of Internal Medicine. Despite the fact that they can be tracked with ereyhromycin, they cause a "doctor's dilemma," he reports; most laboratories, even in the most specialized hospitals, are unable to recognize them.

It took the disease control center months to identify the organism -- now named Legionella Pneumophilia -- that killed 29 people and made 153 others ill at an American Legion Convention at Philadelphia's Bellevue-Stratford Hotel and in its vicinity in mid-1976. The organism wouldn't grow in ordinary laboratory cultures, and new media and growth methods had to be devised.

The new bugs grow in the same or similar media.

"What's strange," Fraser said, "is that we have had so much difficulty growing them, and they seem to grow so easily in cooling towers or lakes. We still don't know why."

Still, Brenner said, most state health laboratories now can isolate the organisms for any hospital, and "any hospital with a good bacteriologist" should be able to identify them.

There probably are 2.5 million cases of pneumonia a year in the United States. About three-fourths are caused by viruses.

Most of the rest are caused by bacteria. But these usually are the most serious cases, and more often prove fatal.

"We think Legionnaire's Disease probably is causing about 25,000 cases a year," Fraser said, "we don't know yet how many of the 'new' pneumonias cause. I'd guess not as many, yet altogether the number may be quite a chunk."

Legionnaire's Disease turned out to be the same, then-unidentified disease that struck at St. Elizabeths Hospital here in July 1965 and in Pontiac, Mich., in July 1968.

At least some cases of an unexplained "fever" at Fort Bragg, N.C., in 1943 are now known to be the same "Pittsburgh pneumonia" that was isolated from the lungs of two Pittsburgh kidney-disease patients in 1979.

So far, Fraser said, there are three main known Legionnaire-like bacteria in addition to Legionella:

"The Pittsburgh agent," also known as TATLOCK (for Dr. Hugh Tatlock of Northhampton, Mass.), HEBA and other names.

WIGA, the first two letters of the still undisclosed first and last names of a Navy skin-diving trainee in Florida who contracted the bacteria in a freshwater tank. Another case was discovered afflicting a fisherman who fell into a brackish South Carolina lake. The same organism has been found in air-conditioning cooling towers. Another name for it, at least until recently, was just MISC. 15.

Tex-KL, isolated from a cancer patient with a typical pneumonia, a common euphemism for unidentified and unresponsive pneumonias, at the M.D. Aanderson Hospital and Tumor Institute in Houston. Tex-KL has turned out to be the same organism as New York 23, found in a cooling tower during a New York City Legionnaire's Disease outbreak that triggered a minor garment center panic in summer 1978.

"Maybe we have a group of water-associated bacteria pathogenic for man," Dr. Fraser said. He said he now believes the Legionnaires and others in or near Philadelphia's Bellevue-Stratford Hotel probably got the disease from water-laden air floating down the side of the hotel from a cooling tower.

But there also have been legionnaires and other LLO outbreaks with no known source, watery or otherwise. And while officials at the Center for Disease Center believe it is prudent to keep commercial air-conditioning towers and condensers clean and bacteria-free, they have no proff that this is effective.

The bacteria have not been found in home air conditioners, and patients have shown no special association with home air conditioning.

Yet in some way, says Dr. Edward Huth, editor of the Annals of Internal Medicine, we are seeing "a new start in uncovering complex relations among man and the myriad microorganisms" of the soil and water.

"My guess," said Dr. Brenner, "is that these are not primarily human pathogens.I think their normal ecological niche is in the environment. But sometimes a quirk happens to concentrate these bugs."

And humans get in the way.