HEMET, CA - APRIL 26: "Sentinel" chickens are kept in areas of concern by the Riverside County Department of Environmental Health to detect the presence of West Nile Virus on April 26, 2007 in Hemet, California. If a mosquito carrying the virus infects a chicken, the viral load will show up in a blood test even though it will not rise to levels that will cause symptoms in the bird. California health officials announced this week that West Nile Virus season is starting earlier than usual because of an unusually warm March. Mosquitoes that carry the virus have begun breeding earlier than usual and the West Nile Virus has been detected in mosquito pools, birds, or horses in eight California counties. Although the virus has not been detected in humans so far this year, 24 people have died and 1,200 sickened by the virus over the past two years in California. West Nile can be transmitted from infected birds, squirrels, and other animals to humans and animals such as horses through several varieties of female mosquitoes. The disease first appeared in the United States in 1999 in New York and killed hundreds of people during its westward expansion before gaining a firm foothold in California in 2004. In reaction to the early start of West Nile Virus season, the California State Legislature this week declared April 23-29 "West Nile Virus and Mosquito and Vector Control Awareness Week". The effected counties include Imperial, Los Angeles, Orange, Riverside, Santa Clara, San Diego, Sonoma and Stanislaus. (Photo by David McNew/Getty Images)