A Chantilly High School student died from an unidentified form of meningitis Thursday, and a classmate, also a 16-year-old girl, was hospitalized with similar symptoms, Fairfax County health officials said yesterday.

Meanwhile, a 12-year-old boy from the Chantilly area died Thursday from a mysterious illness, but health officials believe that his death was not caused by meningitis.

Courtney "Kay" Richard, a sophomore at Chantilly High, spent four days in local hospitals before she died just as classes were letting out for the summer.

Lab tests have revealed little about what type of meningitis she contracted and how she got the disease, leaving health officials mystified, said Kimberly Cordero, spokeswoman for the Fairfax County Health Department.

On one hand, Richard did not appear to have bacterial meningitis, the more deadly form of the disease. But tests for the more common viral meningitis also came back negative.

"Every test that could be ordered has been ordered, and everything has come back negative so far," Cordero said. "Nothing is being ruled out." The Health Department did not release the name of Richard's sick classmate, citing patient confidentiality rules. But Cordero said the classmate's condition has stabilized at Inova Fairfax Hospital.

Meningitis is an infection of the fluid that surrounds the brain and the spinal cord. The symptoms typically are severe headaches, a stiff neck, eye sensitivity to bright light, drowsiness, confusion, nausea and vomiting. It is spread by direct contact with bodily fluids such as the saliva or the mucus of an infected person.

Viral meningitis is generally less severe than the bacterial form, and most people with normal immune systems recover fully after a period of rest.

Bacterial meningitis is life-threatening if not properly treated, but viral meningitis rarely is fatal, said Lucy Caldwell, spokeswoman for the Virginia Department of Health, which is monitoring the situation. "But it can happen," she said.

Health officials are equally puzzled by the death Thursday of the 12-year-old, a Franklin Middle School student. He did not display any symptoms of meningitis before dying abruptly at his home Thursday, Cordero said. An autopsy is scheduled, but it could take up to a week to determine the cause of death.

Neither school nor Health Department officials would identify the student.

His death "is just a total coincidence," Cordero said. "We can't say what the death was [caused by], other than it didn't appear to be meningitis. We know that the child died suddenly as opposed to having symptoms first and then becoming more ill."

Nevertheless, news of the two deaths sent a flood of concern among parents in the Chantilly area.

"Courtney was a beautiful 16-year-old girl and talented volleyball player, talented on the debate team, really a nice kid. She was nice to everyone," said Elizabeth Wright, whose daughter was a friend of Courtney's. "I would just like to know: Where did she get it? . . . It seems very random, and my kids are healthy, but I think as far as we know, Courtney was healthy, too."

Marilyn Perry, president of the Chantilly PTSA, said: "I don't think it's time to panic. . . . I'd like to wait and see what the final decision is from the Health Department. I would rather not overreact until we hear."

With schools following a shortened daily schedule this week, news of the student illnesses and deaths spread slowly.

Outside Chantilly High School, students painted an old concession shed to express their grief. "In Loving Memory," written in large, loopy fluorescent pink letters, appears with a broken heart. One wall bears a painted five-foot portrait of Courtney. Bouquets and notes are scattered in the area.

"Kay . . . wow . . . just last week," a classmate, Julia Arnone, was writing in black marker on the side of the shed yesterday. "She was turning into such a brilliant woman," she said.

Chantilly High School students Buffy Billone, 15, left, and Whitney Frost, 16, attend a memorial for Courtney "Kay" Richard, who died of meningitis.