The region’s warmer-than-average weather has given mosquitoes an early boost, and a couple of Northern Virginia localities are already reporting the first batches of bloodsuckers testing positive for West Nile virus.

Officials with Prince William and Fairfax counties say they have found mosquito pools with the virus, which is carried by birds but can be transmitted to animals and people through the bite of an infected mosquito.

“Typically, we don’t start seeing cases until early July,” said David Gaines, a public-health entomologist with the Virginia Department of Health.

The rainfall of the past few weeks, combined with the warm weather, have been favorable for mosquito development, experts said. That, mixed with a bird population carrying the virus, could increase rates of West Nile transmission.

“When we have a lot of infected birds out there, and then we have high populations of mosquitoes that can transfer that virus from the bird population to the human population, that sets up the recipe for trouble,” said Michael J. Raupp, a professor of entomology at the University of Maryland. The question now, he said, is how large the population of infected birds is.

Local and state authorities have not reported any cases of human West Nile virus this season, but they are encouraging residents to take steps to avoid mosquito bites.

The virus generally takes a bigger toll on birds than humans. It could take anywhere from two to 15 days after being bitten before symptoms — which including a fever, headaches and rashes — appear. The elderly and those with weakened immune systems are more susceptible to infection.

In Maryland, Department of Health officials said they have no human cases of the virus this season, but their surveillance system begins July 1. They also have no reports of mosquitoes testing positive for the virus.

Some Virginia localities have their own mosquito-surveillance programs and have started testing for the virus.

Prince William reported last week that mosquitoes collected in Woodbridge this month by the county Mosquito Control Program tested positive.

Fairfax has tested 578 mosquito pools and identified four of them as positive for West Nile virus, public-safety spokesman Glen Barbour said.

“Our infection rate is less than 1 in 1,000 – which is considered very low,” he said, noting that this week is Mosquito Control Awareness Week.

The disease, which has been troubling the area since 1999, peaked in 2002, when 10 people in the District, Maryland and Virginia died of it. Officials and experts say that as the region continues to have warm weather and rainfall, there will be larger populations of mosquitoes and more-favorable conditions for transmission of the virus.

The best way to avoid the disease is prevention, they said.

“We are basically coming into the season,” Raupp said. “I have noticed the significant increase in daytime and evening biting over the past couple of weeks. So all the pieces of the puzzle are there.

“The best thing to do is empty those breeding sites. . . . Unplug those gutters, get rid of all standing-water sources around the home.”

And when going outdoors, especially at dawn and dusk, when mosquitoes are most active, residents should wear repellant, long sleeves and long pants.

“This will help cut down on the biting and, again, help reduce the risk of contracting one of these mosquito-borne illnesses,” Raupp said.