Dead birds or other animals in 36 of California's 58 counties, have tested positive for West Nile virus, according to the state website tracking the disease, westnile.ca.gov.
Dr. Mark Horton, director of the state's public health department, recommended that people take steps to avoid mosquito bites.
"Even though the likelihood of serious illness from West Nile virus is low for most people, all Californians should take every precaution to reduce their risk of exposure," he said.
Less than 1 percent will become seriously ill, with those older than 50 particularly vulnerable. Recent data also indicate that people with diabetes or hypertension are at greatest risk.
Severe symptoms can include disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. The neurological effects can be permanent.
News source Xinhua