A 51-year-old Baltimore County man who doctors thought might die from legionaires disease two weeks ago, was apparently cured with a common antibiotic drug, according to physicians at Baltimore's City Hospitals.

The disease was confirmed yesterday through laboratory tests. The man, whose name was withheld at his request, was Maryland's third case of the disease.

"We have no idea where he got it from," said Dr. Philip D. Zieve, chief of medicine at City Hospitals.

The two other confirmed cases of legionnaires disease in Maryland, one in Montgomery and one in Prince George's County, were also treated with the same antibiotic drug, and recovered fully.

The drug called erythromycin, is a commonly used antiobiotic, according to Zieve. The Disease Control Center in Atlanta recommends the drug for suspected cases of legionaire's disease.

A spokesman for City Hospital said the Dundalk man was admitted with a temperature of 105 degrees and an infection in part of his left lung, on Nov. 27. Doctors thought he had pneumonia and placed him in isolation on a mechical breathing device in the intensive care unit for treatment, Zieve said.

"But it was a typical pneumonia," Zieve said."None of the usual bacterial organism were present in his blood and he had very severe headaches in addition."

Despite 24 hours of intensive treatment for pneumonia, the infection spread through the man's left lung and infected his right lung.

"We suspected legionnaires disease at that point and began treatment," Zieve said. "We noticed a slight improvement within three or four days but I didn't feel comfortable about him until a week later."

The man does not require further treatment and is off the medication, Zieve said, "but I expect there will be a period of weakness for the next few weeks. In view of his condition when he entered the hospital and the first week here I seriously doubted he would survive."