Health officials said there was no reason for the public to be concerned. Legionnaire's disease, which peaks during summer months, is not spread from person to person. It is transmitted when people breathe mist or vapor from water where the bacteria live, and is treated with antibiotics.
Most reports this year, as usual, have involved single isolated cases, giving epidemiologists no pattern to trace and no reason to suspect an outbreak.
When a maintenance employee who worked near cooling towers at two Philadelphia public schools was recently diagnosed, city health officials, following guidelines, did not recommend any action and said Wednesday there was no reason for parents to worry when schools open Sept. 9.
The district decided to test the tower and heating and cooling systems as a precaution, as businesses often do. Tests for the bacterium that causes the disease were negative for Martin Luther King High School, a spokesman said, and results for McKinley Elementary School are expected soon.
City health officials are intimately familiar with Legionnaire's disease, which got its name after more than 200 people came down with a mysterious pneumonia, and dozens died, at a 1976 American Legion convention at the Bellevue-Stratford Hotel.
Warm water, such as that found in plumbing and many other places, is now known to be an environment in which Legionella bacteria thrive.
Between 20 and 70 cases annually have been reported in the city in recent years, and one to four deaths. Forty-two cases have been reported, but not all confirmed, so far this year, and one person has died. Because the illness causes pneumonia, most diagnoses are made in hospital patients.
Smokers and people with compromised immune systems are at higher risk. Most health departments conduct full investigations when reports are in nursing homes or in clusters, suggesting a common source.
" Legionella is always in water. Heating and cooling systems, puddles . . . but most of the time it doesn't bother people," said Edward Lifshitz, medical director of the New Jersey Health Department's communicable-disease service. Single cases are virtually impossible to trace, he said.
New Jersey confirmed 114 cases through July, the most in a decade, and seven deaths.
The Centers for Disease Control and Prevention, which said that about 10 percent of infections are fatal, has gotten reports of 2,595 cases through Aug. 17, compared with 1,965 during the same period last year. Most cases, as usual, are in the Northeast.
"Several states in the Mid-Atlantic and nearby regions have recently reported an increase in Legionnaires' disease cases either statewide or in certain counties," CDC spokesman Jason McDonald said in an e-mail Wednesday.
Although the reason for the bump in early summer is unknown, he and others noted that research has found a link between environmental factors, such as rainfall, and increased reports of disease.
An analyis of cases in Southeastern Pennsylvania from 1995 to 2003, published in the Journal of Infectious Diseases, found a 21/2-fold increase in cases when there had been precipitation shortly before. (Symptoms, which can begin with fever, chills, and a cough, typically begin within a week or two of exposure.)
Caroline Johnson, a coauthor of that report, is disease-control director of the Philadelphia Department of Public Health.
"We had a busy early-summer season [June and early July], but things have slowed to an average pace now," she said.
Asked if she had any concerns about the Philadelphia schools where the maintenance employee worked, she said no.
"If you want to avoid Legionaire's disease, don't smoke," Johnson said.
Contact Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.