Eighteen dead birds have tested positive for West Nile virus in Sacramento County this year. Statewide, 49 dead birds have tested positive so far. The average for this point in the season is 22. West Nile virus circulates among wild birds and is spread to humans by mosquito bites. People who get the virus, they can't transmit it to others. Sacramento County Health Officer Doctor Phuong Luu says most infections are mild, but the risks grow for older adults and those with chronic conditions.