Reggie Miles finally went home to Seat Pleasant yesterday after a 54-day stay in the Washington Hospital Center, including a month on the critical list.

His departure was duly recorded by the electronic and print news media, a crowd of reporters who wanted to know how he felt when he was told what was making so sick - Legionnaire's disease!

What the 27-year-old GSA employe proceeded to describe was a bad case of pneumonia, for that is all the popularly dreaded Legionnaire's disease is.

What Miles didn't know was how lucky he was: In the week ending Sept. 16, 11 persons died in the District of Columbia of other forms of pneumonia.

Only four persons have died of Legionnaire's disease in the District since 1976.

A common impression has developed that Legionnaire's disease is particularly virulent - a modern plague-like killer, striking at random.

This impression originated with the outbreak in 1976 of a mysterious pneumonia-like disease that afflicted 191 persons attending an American Legion convention in Philadelphia. The disease did not seem to respond to the usual drugs used to treat pneumonia.

As it turned out, only 29 of the 191 persons (15 percent) died of the disease. The usual mortality rate for properly treated cases of the most common form of pneumonia is 5 to 10 percent.

At the time of the Phildelphia outbreak, physicians had no idea what was causing the illness, but by January of last year a previously unidentified bacterium was found to be the culprit.

Scientists at the federal Center for Disease Control in Atlanta also checked the bacterium against frozen blood serum and discovered that so-called Legionnaire's disease was none other than what was in 1966 labeled St. Elizabeths disease after a mysterious outbreak at the federal mental hospital here, bearing that name.

The St. Elizabeths outbreak resulted in the deaths of eight of 62 patients, or 13 percent, who developed pnuemonia the cause of which physicians could not identify.

Since the discovery of the bacterium, numerous scattered cases of the disease have been reported around the country, and there have been several more major outbreaks, including one this summer in a five-block area of New York's garment district.

What is particularly frightening about Legionnaire's disease, as opposed to the more commonly identified forms of pneumonia, is that cases of it have appeared in tight clusters including those in New York, Philadelphia, Bloomington, Ind., and at St. Elizabeths.

The disease does not appear to spread from human to human, doctors say, even though persons in the same environment, be it a hotel or work place, seem to contract the disease during the same time period.

Scientists now believe the bacterium may be found in dust and perhaps in damp places. In Bloomington, the site of infection was traced to an air conditioning duct system, and CDC investigators were checking air conditioners and puddles in subway tunnels during the recent New York outbreak.

There was no apparent connection, however, among the four patients who contracted Legionnaire's disease in Washington this summer. The summer is simply the season for Legionnaire's disease, doctors say.

Reggie Miles told those attending his press conference yesterday his illness began with sharp stomach pain "that wasn't like a normal pain."

According to Dr. James Curtin, chairman of the department of medicine at the Hospital center and an authority on infectious diseases, Miles' abdominal pain is a hallmark of Legionnaire's disease, and it is also one of the things that warns physicians they may be dealing with the mystery disease rather than another form of pnuemonia.

Within 24 hours of first feeling sick, Miles was hospitalized with a high fever and other classic symptoms of pneumonia, which is itself a group of symptoms, rather than a specific disease, caused by a large number of bacteria and viruses.

Like many pneumonia patients, Miles had to be placed on a respirator to help him breathe. One of the unusual aspects of his treatment, however, was that physicians had to give him paralyzing drugs because he fought so hard when they first attempted to hook up the machine. He remained paralyzed for about two weeks.

Miles was treated with the drug Erythromycin, an antibiotic that is proving to be relatively successful in combating Legionnaire's disease, Curtin said.

He said that as more hospitals treat serious cases immediately with Erythromycin before waiting to identify the cause positively through laboratory work, the death rate for Legionnaire's disease will probably drop. "The mortality rate is around 4 percent for the patients who get" the antibiotic, he said.