Seven birds and a group of mosquitoes have tested positive for West Nile virus in Contra Costa County, with the majority of the activity in El Cerrito, health officials said Friday. El Cerrito has been the site of only one previous positive bird test since the Contra Costa Mosquito and Vector Control District first found the virus. Mosquitoes typically become infected with West Nile after biting an infected bird. El Cerrito residents should put on mosquito repellent before going outside in the morning and evening, when mosquitoes are most active, the district said. The district also recommended that Contra Costa County residents check their front and back yards at least once a week and dump out any standing water where mosquitoes can reproduce.