OCEANSIDE -- A group of mosquitoes trapped near Oceanside Boulevard and El Camino Real has tested positive for the West Nile virus, San Diego County health officials said in a letter sent to potentially affected residents last week.

Technicians are treating the area with chemicals and surveying the neighborhood to determine where the mosquitos might be breeding, the letter states.

It is not clear how many letters the county sent or when the infected mosquitoes were trapped. Live mosquitoes are trapped at various locations each week and tested for the virus, health officials said.

At least three other groups of mosquitoes trapped in Oceanside and two in Del Mar have tested positive for the virus this year, according to the county's Web site.

West Nile virus is a potentially serious illness that can be spread from birds to humans by mosquitoes. So far, four human cases have been reported in San Diego County this year, officials said. All four people were hospitalized with encephalitis.

Some 262 dead birds found all over the region have also tested positive for the virus. County officials have urged people to avoid getting the virus by wearing protective clothing and insect repellent when outdoors, particularly at dawn or dusk, when mosquitoes are most active.

Any standing water sources where mosquitos can breed should be eliminated and door and window screens should be in good repair, officials said.