Two things coexisted peacefully Thursday afternoon inside the West Chester University library - senior Janna Wilson and, in the cooling system, the bacteria that cause Legionnaire's disease.

"I'm not worried," said Wilson, studying a day after officials notified the campus community that an employee had contracted the disease and that eight campus buildings - including the one that houses the library - had unacceptable levels of the Legionella bacteria.

The positive tests for the bacteria do not necessarily mean the employee got it at the university or that others are at risk, experts said.

"If there is just a single case, then I wouldn't think it should concern people," said Paul Edelstein, director of the clinical microbiology lab at the Hospital of the University of Pennsylvania and a professor at Penn's Perelman School of Medicine.

Legionnaires' is a potentially deadly form of pneumonia caused by Legionella bacteria, which grow in water. People contract it by inhaling water droplets or mist, often through air-conditioning systems, hot tubs, or fountains. It is not contagious.

The unnamed employee's illness was reported to the university July 6 by another worker, and the tests on campus buildings were conducted July 9, the university said. Those results came back July 22 and the school notified Chester County officials Monday.

But they weren't able to talk to the sick employee and confirm his diagnosis until Wednesday, university spokeswoman Pam Sheridan said. Once they did, they alerted the rest of campus in an e-mail.

"It wasn't as if people were particularly at risk," she said.

Legionnaires' disease got its name from a 1976 outbreak at Philadelphia's Bellevue Stratford hotel, which was hosting a state convention of the American Legion. More than 200 people were sickened and 29 died.

Amid national fear of an epidemic, health-care professionals worked to identify the then-mysterious illness. In January 1977, the Centers for Disease Control said it was the previously unrecognized Legionella bacterium, circulating in the air-conditioning system, that had killed and sickened so many.

Outbreaks aren't uncommon. One in the South Bronx this month killed two people and sickened 31, New York City officials said Wednesday. Nearly two dozen other cases across that city had been reported in the last nine months.

It is fairly common for the bacteria to grow in water, but Legionella only sickens humans under certain conditions. Because of its prevalence - Edelstein estimated that most of the cooling towers in Philadelphia would test positive - where someone contracts the disease can remain a mystery.

The disease, which is treated with antibiotics, often crops up during the summer, when there is high humidity.

Symptoms usually appear two to 14 days after exposure and include coughing, fever, headache, and muscle aches, according to the CDC. It places the chance of death between 5 percent and 30 percent, with higher risks for the elderly, smokers, or those with immune problems.

There was no palpable sign of concern Thursday on West Chester's campus, which during semester peaks counts more than 14,000 students and staff. Students preparing for finals in summer classes shut themselves away in libraries. Kids participating in camps meandered across the campus.

"If I had to go to that campus to pick up a child or give a lecture or see a concert, I would go without a second thought," said Peter Axelrod, a Temple University professor of medicine and expert in infectious disease.

Unless an epidemic breaks out at West Chester, there is no way to know for sure whether the employee got the disease at the university without doing genetic testing.

"It's certainly possible he picked it up there, but to me it's by no means certain," Axelrod said.

At West Chester, cooling towers at Sykes Student Union, Main Hall, Lawrence Hall, Merion Science Center, Schmucker Science Center, Francis Harvey Green Library, 201 Carter Dr., and the School of Music and Performing Arts Center tested positive for "higher-than-acceptable" levels of the bacteria. Each was being treated.

Sheridan said the university would continue to monitor the towers to make sure the bacteria do not return.