Public health officials in Arlington County last week reported finding mosquitoes infected with the West Nile virus in two traps in the county's northern region.

"Finding these [infected] mosquito pools is important because we know the virus is active now," said Aftab Hussain, vector control supervisor at the Arlington County Environmental Health Bureau. This means the chance that people will get bitten by infected mosquitoes has increased, he said.

The West Nile virus is a potentially serious illness transmitted to humans mainly by mosquitoes. It usually peaks during summertime and continues into the fall. Since appearing in the United States in 1999, 16,000 human cases of West Nile virus have been reported and there have been about 400 deaths, according to the Centers for Disease Control and Prevention.

Most of those fatalities have been concentrated in the New York region. In the District, Maryland and Virginia, there have been 29 West Nile deaths since 2002.

Health officials report that the disease has had a delayed start this year due to heavy rainfall and an extended cold season. The slow start "is certainly good news," said Kimberly Mitchell, an entomologist at Center for Veterinary Public Health at the Maryland Department of Health and Mental Hygiene. "Normally by this time we would have had several positive mosquito pools" trapped.

"But it's too early to tell how the season will end up," said David Gaines, the state public health entomologist at the Virginia Department of Health.

According to the Centers for Disease Control and Prevention (CDC), 80 percent of those bitten by infected mosquitoes show no symptoms. The other 20 percent display flu-like symptoms, and about one in 150 of them will develop severe illness, such as meningitis or encephalitis.

Mike Raupp, professor of entomology at the University of Maryland, College Park, emphasized that West Nile virus is serious, especially for the elderly, people over 50 and people whose immune systems are weakened due to disease or medications. "But that doesn't mean others aren't going to get affected by the illness," he added.

In addition to infected mosquito pools, there have been some reports of dead crows in Maryland, although health officials there do not collect data on infected birds or mosquitoes. Crows are one of the carriers of the virus that die from the disease, and their death in large numbers is seen as a marker that the virus is active in the area.

"There is a public health concern [about West Nile virus], but we have that concern without [the birds'] dying," said Marilyn Piety, the West Nile virus coordinator at Montgomery County Public Health Services.

Birds are the main reservoir for the disease. During summer, very high levels of the virus are found in birds such as robins, songbirds, crows and blue jays. The mosquitoes thriving in the same hot weather pick up the virus when feeding on infected birds. They transmit the virus when they bite humans afterwards.

Programs for mosquito control, survey and elimination are usually handled by local health authorities and overseen by state health departments. There are no treatments for West Nile disease, since like cold and flu, it is caused by a virus. Patients who experience milder flu-like cases are usually given medications to alleviate their symptoms.

West Nile also can be transmitted through blood transfusions. In an article published last week in the New England Journal of Medicine, researchers report that "routine testing in 2003 and 2004 identified 540 [blood] donations that were positive for the West Nile virus RNA," the genetic material of the virus. This led to the removal of infected blood from the American Red Cross supply. Since the middle of 2003, virtually all blood donations in the United States have been tested for the virus.

Nationally, 109 human cases of the disease and three deaths have been reported to the CDC so far this year. Western states have reported the highest numbers and officials speculate that the virus may be migrating in that direction, perhaps explaining why numbers have stayed low in the east. The western region of the country was unaffected by West Nile during the virus's first few years in the United States.

According to the CDC, there are no documented cases of transmission of the virus from domestic animals to humans. "We haven't seen any known cases [of the disease] in cats or dogs, but there may be a few per year around the country," said Raymond Phillip, a veterinarian at Friendship Hospital for Animals in the District. He added that, if infected, cats and dogs will not transfer the disease to their owners, though there is a "remote possibility" a mosquito could transfer it from pet to owner, he said.

For most, a West Nile infection will be a one-time event.

"The conventional wisdom is that if you've been exposed to it, you'll be immune to it," said Gaines. "We don't recommend people to become exposed, though."