A 27-year-old Silver Spring man, now at home and well, became the area's fifth apparent victim of legionnaire's disease last month, health officers said yesterday.

Three confirmed District cases, one of them fatal, and another "highly suspicious" fatal case of this disease - now known to be one form of bacterial pneumonia - were reported on in The Washington Post on Sept.e 29.

All the District cases occurred either in June or August, and they and teh new Maryland case "just represent the sporadic instances of thsi disease we're now seeing all over the country," Dr. Martin Levy, director of the D.C. communiacble disease agency, said yesterday.

"The more we look, the more we find - they're nothing unusual any more and nothing to alarm the public about," he added, pointing to the latest figures from the federal Center for Disease Control.

There have been 51 scattered cases, 12 of them fatal, in 22 states and the District so far in 1977, as well as three clusters or outbreaks of the illness: 23 cases so far and 13 deaths in a Vermont outbreak that has apparently subsided; 13 cases and three deaths in Tennessee; nine cases with one death in Ohio.

The disease was first identified in summer 1976 when it hit 180 persons and killed 29 among those who attended a Philadelphia American Legion convention. The bacterium that causes it was identified in January, and scientists found the same "bug" had caused a 1965 epidemic at Washington's St. Elizabeths Hospital.

"Apparently this disease is endemic" - around all the time - "and sometimes it causes epidemics too," Dr. Stephen Thacker, CDC officer, explained.

But in both cases it is still a form of pneumonia, a disease caused by various bacteria, viruses and other microorganisms and, in about half to fall cases, by some still unidentified agent.

In the District a 47-year-old man got the disease in June and died in August; a man, 55, was hospitalized in August and released last month; and a Washington man, 80, became ill and died in June.

Laboratory studies are incomplete in the last case, as well as in the newer one in Silver Spring, but the District case is called "highly suspicious" and the Maryland case "highly probable" by the doctors.