Thirty-nine cases of human West Nile virus have been confirmed in the state as of Tuesday. There have been no fatalities related to West Nile virus in California this year, but officials fear an outbreak between August and October, the peak period for infections.

Humans and animals contract West Nile virus from bites of infected mosquitoes, which contract it by feeding on birds that have the virus in their blood. Most people who are infected by the virus do not become ill, but one in five people develop flu-like symptoms. In its most severe form, West Nile can cause encephalitis, meningitis and death.

This year, 70% of the victims have contracted the more serious neurological form of the disease, West Nile neuroinvasive disease, Cummings said. That's a higher number than in past years, but officials do not know why that is. It's an additional cause for concern, officials said.

Southern California has seen an increase in the number of infected dead birds and mosquitoes this year. So far, 566 dead birds and 316 mosquito samples have been confirmed positive for the virus in the area.