NORTH COUNTY -- Planning to spend the weekend outdoors? Better pack the bug repellent. County health officials say it's more important than ever for people in San Diego County to protect themselves from mosquitoes -- and the potentially deadly West Nile virus they can carry.

The virus, which makes relatively few people sick but can be fatal in some cases, has shown up in five dead birds throughout the county this summer. Four were found earlier this week in Escondido, Vista, Fallbrook and Rancho Penasquitos.

Finding the infected birds is generally a precursor to finding the disease in humans, and environmental health workers say they're certain the virus will infect people in San Diego County.

Mosquitoes carry the virus from birds to humans when they bite.

"The pattern in other areas is that infected birds are found, and then you start seeing human cases," said Jack Miller of the San Diego County Department of Environmental Health.

"In L.A. and San Bernardino counties, they were getting up to 20 dead birds per week and then they started seeing human cases," Miller said. "It's pretty clear the virus is becoming more predominant here."

Most people who are infected with the virus don't become ill, and county health officials said there is no reason to panic now that the virus is present in San Diego animals.

West Nile virus causes mild, flulike symptoms in about 20 percent of people who are infected and serious symptoms in fewer than 1 percent, according to the Centers for Disease Control. The virus has killed 270 people in the last two years. More than 10,000 people have reported becoming ill from the virus, according to the federal agency.

So how do people protect themselves from the disease?

Health officials say the first step in keeping the virus at bay is getting rid of standing water, where mosquitoes like to hang out and breed. They suggest dumping out birdbaths, water pails, old tires and anything else that could collect water.

"If you've got a kid's toy out there that keeps filling up with water every time you water your lawn, dump it out," said Miller. Mosquitoes can breed in less than a cup of water, he said.

Swimming pools should be covered or properly maintained -- ponds and pools that do become stagnant should be stocked with mosquito-eating fish.

The county has spent the last three months dropping larvicide in lakes and lagoons, trying to kill mosquitoes in those areas before they get old enough to breed.

"The fact is that this county has a lot of water sources. It's one of the nice things -- lots of water and lots of open space," Miller said. "Unfortunately, that also means we have the environment that gives us the potential for this illness to be upon us."

Anyone who spends time outdoors -- especially near water or at the dawn and dusk hours -- should wear an insect repellent containing diethyl-m-toluamide, more commonly known as DEET, Miller said. Long pants and long sleeves are also good ways to keep mosquitoes from biting.

The department also suggests that anyone who finds a dead bird call the county so officials can test the animal for the virus.

More information on the virus is available at www.sdfightthebite.com or by calling (888) 551-4636.

Contact staff writer Erin Walsh at (760) 739-6644 or ewalsh@nctimes.com.