A Loudoun County teenager is being treated for malaria, and health officials believe she likely was infected last summer at the same time two other county residents contracted the mosquito-borne illness.

The teenager, who was hospitalized and has responded well to medication, began suffering from a fever and chills March 12, Loudoun health officials said. They suspect that the parasite that causes malaria had remained dormant in her body and that she was infected months ago in the same eastern Loudoun neighborhood where two other teenagers likely were bitten by infected mosquitoes.

"We can't guarantee that's what we'll find, but that seems the most likely answer," said David Goodfriend, director of the Loudoun County Health Department. "If not, it's just an amazing coincidence that she was right at the same spot as the other two. All three were within a block of each other on the same night."

Goodfriend said tests have confirmed that the three people had the same kind of malaria -- the vivax strain, which is not deadly. Blood samples from the three teenagers are being tested at a state laboratory in Richmond as well as the Atlanta-based Centers for Disease Control and Prevention to determine whether the strains are identical and likely originated at the same time and place, he said.

The emergence of the teenager's symptoms comes as officials around the Washington region are preparing for the onset of warm weather, which will bring the danger of mosquito-borne illnesses, including West Nile virus. In Loudoun, health workers already are testing hibernating mosquitoes for infection and are preparing to reduce the mosquito population by killing larvae.

The strain of malaria found in Virginia is the mildest form of a disease that afflicts hundreds of millions of people around the world every year and is easily treated with proper medication. Locally contracted cases are rare in the United States, and health officials believe the three Virginia teenagers were bitten by infected mosquitoes that had sipped blood from someone who contracted malaria overseas.

Most people who are infected become sick in two to three weeks but the onset may be delayed for up to 10 months, health officials said. John MacArthur, a CDC epidemiologist, said it's not unusual for several months to go by before a person who has contracted malaria becomes ill. A long delay does not result in more severe symptoms or long-term health problems, he said.

Goodfriend said the teenager visited her doctor after she began feeling poorly and, after antibiotics didn't help, went to an emergency room. A physician at the hospital recognized the symptoms, and tests confirmed she had contracted malaria, Goodfriend said. He said that the teenager was feeling better and that doctors expected to release her from the hospital yesterday.

When health workers questioned the teenager, they learned she had spent time in the same eastern Loudoun suburb where officials believe the other two teenagers were infected last summer, Goodfriend said. He said the teenager has not traveled recently and does not recall any recent mosquito bites.

"When we march back in time, if this is not a dormant infection, then the mosquito would have had to have been out just after that big snowstorm," said Robert Wirtz, chief of the CDC's entomology branch. "That evidence suggests this was a delayed infection."

Goodfriend said the two teenagers who were infected last summer, a high school boy and female college student, have fully recovered. Health officials did not identify any of the teenagers.