PLACER COUNTY, Calif. -- Recent Northern California storms may have moved on, but they left behind a lot of standing water, which is proving to be the perfect breeding ground for mosquitoes, causing more concerns over the West Nile virus.

WEST NILE VIRUS Interactive: All About West Nile Overview: Learn About Disease Protection: Insecticide Tips More: Virus In Horses RESOURCES Encyclopedia: West Nile Defined Facts: CDC | Q&A Maps: Cases By State | Cases By County Phone CDC Hot Line: (888) 246-2675

More than a dozen Northern California counties have detected the West Nile Virus this season. Experts say all the water sitting in the valley, plus the recent warm temperatures, create the perfect conditions for mosquito eggs to hatch.

"Anything that will stand for more than seven to 10 days, that's the life cycle of a mosquito," said Sac-Yolo Mosquito Vector Control spokesman David Brown.

Horse owners are also taking extra care to protect their animals from the West Nile virus. Last year, more than 500 horses died from the virus.

"What we're asking people to do is to start taking precautions now, establishing good habits to protect yourself from mosquito bites," said Placer County vector ecologist Dr. Jenesina Scott.

Experts say to use lotions with DEET to keep mosquitoes away. And there are even clothing lines that are designed to repel mosquitoes, including a line called Buzz Off.

Doctors say that senior citizens are the most vulnerable to the West Nile Virus.