"Why us?" asked Mary Stowell, a cleaning woman. "I can see why there'd be a disease like this in a big, dirty city like Philadelphia. But why us?"

This clean, quiet state of autumn yellows and browns and low, snow-tipped mountain peaks is the site of what has proven to be the largest fatal outbreak of "Legion Fever" since the one last summer in Philadelphia.

It is also the feverish new site of what one scientist has called "one of the major epidemiological searches of the century" the search for an answer to Mary Stowell's "why."

For nine months, scientists have known that the fever is cased by a bacterium, a germ.

Now they are narrowing their probe in an effort to find out where the germ usually hides, how it spreads and what its habits are.

"Epidemics are experiments of nature," Dr. Theodore Tsai of the Center for Disease Control, headquartered in Atlanta, said last week. "That's why what's happening in Vermont and elsewhere has us excited. We feel we have a good chance now to find some answers. We're getting right in on the action."

The "action" in Vermont, Tennessee and Ohio - all sites of current or recent outbreaks - as well as more than 50 other cases in 19 states since January have told investigators these things.

Legion fever is not a complete "mystery" ailment, but one of some 100 forms of pneumonia, which simply means inflammation and congestion of the lungs.

It is a broad classification like heart disease, which also has many forms and causes.

About a quarter of the 2.8 milliom cases of pneumonia in the United States each year are caused by well-known, identifiable bacteria. Another quarter is caused by viruses or microorganisms called mycoplasma.

But the organisms that cause about half of all pneumonia remain unknown. The Legion bug has been uncovered as one of these culprits, causing possibly as many as 1 or 1.5 per cent of all pneumonia.

This is the estimate of some CDC officials and Dr. Charles Phillips, head of the infectious disease unit at the Vermont Medical Center Hospital, the teaching hospital of the University of Vermont.

But this small percentage, if it is correct, could mean between 28,000 and 42,000 cases a year, with a death toll of 4,000 to 7,000.

"The Legion disease is not a new germ at all," said Phillips. "It is just one that had never before been recognized."

Medical science, ever more sophisticated, frequently finds new diseases. Last year a new bacterium was dicovered in wounds caused by dogbites. CDC scientists since the late 1960s have identified and isolated the viruses that caused three major, previously unknown diseases in Africa: Lassa Fever, Marburg Fever and Ebola Fever.

"We're finding new diseases often," said Dr. David Fraser of CDC.

Legion pneumonia has been 15 to 17 per cent fatal in most outbreaks. This was true in Philadelphia (where 181 legionnaires or others were ill and 21 died) and in what has been identified as the nation's first positively known outbreak, one at Washington's St. Elizabeth's Hospital in 1965.

But 144 public health employees got what was either Legion fever or a close cousin in Pontiac, Mich., in 1968. They developed respiratory illness, but not full-blown pneumonia, and none died.

Doctors seem to be finding that most cases of Legion fever can be successfully treated with the antibiotic erythromycin.

In Vermont the death toll is 13 out of 19 confirmed or highly probable cases. But the disease was first spotted after an unusual cluster of pneumonia fatalities at the Vermont hospital.

Five CDC investigators and Vermont health officials are scouring the state for other current or past cases.Spokesmen for the hospital said yesterday that tests indicate the disease was present in the Burlington area before the current outbreak. The preliminary tests were made on blood samples from people who reported a pneumonia-like illness before August.

Surveillance this year in six states, as well as the outbreaks, have revealed that there are some Legion disease cases around all the time, but there also are the epidemics, perhaps seasonal, just as winter always sees a climb in pneumonia triggered by influenza virus.

"The honest-to-goodness truth" is that there's Legion disease all over the country and "we're being penalized for being good detectives" at this hospital, Phillips said.

Yet he also pointed out that most of the known Vermont cases, unlike the cases among the relatively healthy legionnaires, suspiciously occured at the hospital itself.

Of 19 cases, he reported, 14 apparently were infected in his hospital, considering that the disease's incubation period is about two to 13 days.

Most were physically run down, of course, or they wouldn't have been in the hospital in the first place. Of the 13 persons who died, four had cancer, three had had kidney transplants, two had been on kidneys dialysis and one was being given steroids. So all these persons had impaired immunological defenses. And the other three deaths were among persons 55 or older.

Still, Dr. Stephen Thacker - CDC officer in the District of Columbia health department, currently part of the Vermont investigating team - said Friday that the Vermont outbreak is "clearly not a hospital problem" but something broader in the community.

CDC officials are nonetheless looking at hospitals or health centers as a possible focus since the hospital or neighborhood involved has produced, at the least, a suspicious number of cases in Kingsport, Tenn., Columbus, Ohio, as well as in Vermont. The Pontiac outbreak was at a health center, and then there was the St. Elizabeth's epidemic.

The CDC investigators are looking at public buildings, whether hospitals or hotels. It was learned in August that in 1974 ther was an apparent outbreak at Philadelphia's Bellevue-Stratford Hotel, the site of the 1976 American Legion convention.

And last week CDC doctors learned that in 1973 the same bug apparently infected 86 Scottish tourists staying at a hotel in Benidorn, Spain, killing three. This year the same infection was found in the blood of another Scot just returned from the same city.

The diesase is "probably airborne" rather than passed from person to person, said CDC's Fraser. But he admitted that "we say this largely because other modes of transmission like person-to-person or food or water have so far been ruled out."

If transmission should prove to be person-to-person, the bug must be very particular. None of the roommates or family members of the legionnaises got the illness.

"It is a fastidious bug, hard to grow, but then hard to kill," said Dr. Joseph McDade, who isolated the germ. "There's a lot left to learn about it."

Maybe, said Phillips, it is a germ that lives inside many of us all the time, but becomes active only when our defense are somehow weakened.

"Maybe," said Tsai, "we will never know all the answers. That remains true in many diseases. But I really don't think that will be the case with this disease."

In Vermont, Tennessee and Ohio, CDC officers and volunteers have been asking scores of persons questions - "Have you been ill recently?" ""Where have you traveled?" - to try and get more information about the germ.

"People here are eager to cooperate. They haven't panicked," Phillips said. "They're taking it all very calmly. Maybe that's Vermont philosophy."

It's probably more. An Allegheny Airlines plane from New York City Thursday night was jammed withe youths with boots and backpacks, ready to hike into the woods and hills to see Vermont's violently beautiful autumn colors.

"Are you worried about Legion disease?" a young woman was asked.

"What's Legion disease?" she replied.