Local health officials still cannot identify the virus that caused three cases of meningitis -- one fatal -- last week in the Chantilly area.

The disease killed a Chantilly High School sophomore and required another 16-year-old girl from the same school and an elementary school teacher to be hospitalized, authorities said.

Lab tests have ruled out several common viruses. But they can be hard to identify, said Denise Sockwell, a state epidemiologist. "We may never know what kind of virus caused this," she said.

The three infections are the only confirmed cases of viral meningitis currently in Fairfax County, and Fairfax's health department is not actively investigating any others, said Gloria Addo-Ayensu, the agency's director, who sought to allay residents' concerns yesterday afternoon at a news conference.

"It's not unusual to see several cases of viral meningitis at this time of year," Addo-Ayensu said. "For viral meningitis to cause death is rare. That is why the death of a 16-year-old has shocked all of us."

Courtney "Kay" Richard died Thursday after being hospitalized for four days. Another girl was hospitalized with the disease Thursday. Asked if there was a connection between the two cases, Addo-Ayensu said that the girls "weren't best friends . . . but they went to the same school."

A 12-year-old boy who attended nearby Franklin Middle School in Chantilly died suddenly at home on Thursday, but authorities said they do not believe that meningitis was to blame. A county police spokesman said foul play also was ruled out. Health officials have said an autopsy could be completed as early as this week.

And on Friday, a teacher at Reston's Armstrong Elementary School was hospitalized, becoming the third confirmed case of viral meningitis last week. But doctors have not found any link between that case and those of the Chantilly girls. The teacher and the 16-year-old are in stable condition at Inova Fairfax Hospital, authorities said.

Meningitis, an infection of the tissue surrounding the brain and spinal cord, can be caused by viruses or bacteria. Symptoms included headache, stiff neck, sensitivity to light and drowsiness. Lab work indicated that none of the three contracted bacterial meningitis, the disease's more deadly form, Addo-Ayensu said.

Because many viruses can cause meningitis, knowing the source is important for deciding how to prevent its spread, she said. Infection is most commonly spread by close contact, such as sharing food, kissing or even using the same tube of lip balm. But viral meningitis also can be transmitted via mosquitoes or by swimming in a very poorly filtered pool, she added.

The county health officials said they received several calls from concerned residents during the weekend after news of the three infections broke.

She said she decided to hold a news conference yesterday because she wanted to spread the facts about viral meningitis. Most people recover quickly from it, and some may exhibit such mild symptoms that they don't even know they have the disease.

In fact, state health officials considered viral meningitis harmless enough that in 1999 they dropped it from the list of "reportable" diseases, meaning doctors no longer had to notify their local health department if they diagnosed a case, according to Sockwell, the state epidemiologist. Bacterial meningitis, however, remains on that list.

"If it weren't for this death, [viral meningitis] may have not gotten this much attention," Addo-Ayensu said.