CONCORD (CBS SF) — Two groups of mosquitoes in separate Contra Costa County communities tested positive for West Nile virus, officials with the Contra Costa County Mosquito & Vector Control District confirmed on Friday.

One group of infected mosquitoes was found near the unincorporated community of Knightsen and the other in Antioch.

The infected mosquitoes, if they bite humans, can pass along the virus, which can cause death.

Two county residents died from the disease in 2006. Since 2005, 55 residents have been diagnosed with the virus, district officials said.

The officials are encouraging residents to wear mosquito repellent to reduce the chance of being bitten. Effective repellents are those with DEET, Picaridin or Oil of Lemon Eucalyptus.

Residents can take other precautions as well such as dumping or draining standing water and maintaining swimming pools.

A neglected swimming pool can produce more than one million mosquitoes and affect areas up to five miles away.

Mosquitoes can’t start their lives without water, according to district officials.

Residents can also reduce their risk of getting the disease by staying indoors after dusk and before dawn when mosquitoes are most active.

This is the third time West Nile virus has been detected in the county. Earlier this month three birds in Antioch and two birds in Brentwood tested positive.

The first indication the virus was present in the county came in June when a bird in Antioch tested positive.

For current West Nile virus activity in the county visit http://www.contracostamosquito.com. Information about human cases of the virus can be obtained by calling Contra Costa Health Services at (888) 959-9911.

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