SANTA CLARA (CBS SF) — Five residents of Santa Clara County have been infected with the West Nile virus, including two who developed the severe neuroinvasive form that can lead to death, a county spokeswoman said.

Those with the virus in the county live in areas where the highest concentration of mosquitoes carrying West Nile were found this year, including Mountain View, Sunnyvale, Santa Clara, Cupertino, Campbell and west and south San Jose, Valley Medical Center spokeswoman Joy Alexiou said.

READ MORE: Unique Twist To Pandemic Shutdown Of Long-Establish Santa Clara Restaurant

The cases mark an increase from last year, when two people in the county suffered from West Nile and are the most since the five detected cases in 2006, county officials said.

Three people died from the virus this year in California, but no deaths have ever been reported in Santa Clara County, Alexiou said.

The people infected with the virus in the county this year live in areas where the county’s vector control district have sprayed the most insecticide to kill the adult mosquitoes this year, she said.

Of the five residents with the virus this year, two had the severe neuroinvasive form, called West Nile encephalitis that attacks the body’s nervous system, while one had West Nile fever symptoms and two had it but without any symptoms, Alexiou said.

Those who run the highest risk of contracting the dangerous neuroinvasive form of West Nile, that can kill, “are elderly and people with certain chronic conditions, like diabetes,” Alexiou said.

READ MORE: Man Found Dead In Bullet-Riddled Vehicle In Union City

The West Nile virus is transmitted to humans and animals from the bites of mosquitoes carrying the virus, but the risk of becoming seriously ill is very low, as only less than 1 percent of people infected contract encephalitis or meningitis, according to Alexiou.

The county this year has seen an unusually high number of birds that have died from West Nile virus, with 648 birds so far, which amounts to half all birds that reportedly died from the virus within all of California in 2014, according to Alexiou.

Of the dead birds found and tested in the county in 2014, 85 percent were infected with West Nile, which means that the risk of humans contacting West Nile is higher this year, county officials reported.

The county recommends people wear long pants and sleeves at night when mosquitoes are the most active and use insect repellants when going outside, such as DEET, picaridin, oil of lemon and eucalyptus.

Residents should also make sure to keep doors and window screens closed to keep the insects out and remove standing pools and containers of water, including pet dishes and birdbaths, where mosquitoes breed, officials said.

MORE NEWS: SF Restaurant Apologizes for Denying Service to Armed, On-Duty Police Officers

© Copyright 2014 by CBS San Francisco and Bay City News Service. All rights reserved. This material may not be published, broadcast, rewritten or redistributed