State health officials say the number of people infected with the West Nile virus is now 249 — an increase of 32 percent since earlier this week.
The California Department of Health Services says the mosquito-borne illness has now killed six people in Orange, Long Beach, Los Angeles and San Bernardino counties.
Authorities say the elderly are the most vulnerable.
The virus has been detected in 41 of 58 counties in California.
Human cases have been found in 13 counties.
West Nile virus is carried by birds but only transmitted to humans by mosquitoes.
The virus has killed at least 20 people in the nation so far this year.
The U-S Centers for Disease says California has the highest toll followed by Louisiana, which has reported five deaths.
Officials recommend avoiding mosquitoes by eliminating pools of stagnant water, wearing long sleeves if outside at dusk or dawn, and using mosquito repellant that contains the chemical known as DEET.
About 20 percent of those bitten by an infected insect show flu-like symptoms, while about one in 100 suffer the most severe form of the illness.