West Nile virus is transmitted to humans and animals by mosquitoes and infection rates are highest during the summer months, officials said. Human infections have been reported in 37 California counties so far this year.
"It is an important reminder to take precautions against mosquito bites," Marin County Public Health Officer Dr. Matt Willis said in a statement.
Residents are encouraged to avoid mosquito bites by using a repellant and wearing protective clothing, especially during peak mosquito activity hours at dawn and dusk. Windows and doors should have tight-fitting screens without holes to keep mosquitoes outside.
In addition, all standing water should be eliminated or treated to prevent mosquitoes from breeding.
While most people who are infected will not become seriously ill, a small number are at risk of serious neurological illness such as encephalitis or meningitis, officials said. Those most at risk include people older than 50 and those with diabetes or hypertension.