The number of new cases of a pneumonia-like illness believed to be Legionnaires' disease has declined sharply in Charles County, a sign that the baffling, six-week epidemic may be over, Maryland health officials announced today at a news conference.

During the first 10 days of June, 15 cases were reported, compared with 28 in the last 10 days of May, according to Dr. Feng-Ying Lin, chief of clinical epidemiology for the state.

"We had an outbreak. It appears the outbreak is over," Lin said.

Despite their optimism and an investigation that Lin termed "tedious," officials said they had not traced the source of the disease.

Legionnaires' disease is spread when people inhale water drops tainted with legionella pneumophila, a bacterium that was unknown until an 1976 outbreak at an American Legion convention at a hotel in Philadelphia. That outbreak was fatal to 26 of the 182 people who became ill. In many of the more than 30 outbreaks since, the disease has been tracked to bacteria-laden hotel or hospital air-conditioning systems or shower heads.

Aided by specialists from the national Centers for Disease Control Epidemic Intelligence Service, state health officials and environmental inspectors have scoured medical records, tested patients' blood and phlegm and examined air and water samples from homes and public places in the mostly rural Southern Maryland county.

Officials have confirmed as Legionnaires' disease only one case, in which a 61-year-old Charles County woman died May 29. Tests on 66 persons, including a 44-year-old woman who died May 27, have remained inconclusive, officials said. Of those cases, two were described today as "highly probable cases of Legionnaires' disease."

"It's not like it started in one place and spread out," Lin said of the outbreak, which was scattered throughout the county. "Still, today I am leaning toward Legionnaires' disease."

The blood of people who became ill will be retested in about three weeks, officials said, because antibodies against legionella accumulate slowly. Investigators also will look for antibodies against viruses, fungi and other bacteria.

Today, officials said they had eliminated as a likely source Physicians Memorial Hospital in La Plata, where most of the victims have been treated. At one point, one of every five patients at the hospital was suspected of having the disease, which is characterized by a high and rapidly rising fever, chills and a dry cough.

"People got sick at home and then went to the hospital," said Dr. Stephen Redd, an epidemiologist at the CDC.

Lin recounted the case of a 51-year-old, otherwise healthy Charles County man who developed a high fever and chills and almost died. When he was given the antibiotic erythromycin, a treatment that reduces the chance of dying from Legionnaires' disease from 30 percent to 5 percent, he recovered in two days, Lin said.

Officials said they are looking for cases in neighboring counties.

Lin said the investigation found that half of the cases affected people aged 20 to 59, while 36 percent of the victims were 60 or older. Twice as many men as women were stricken.