The first person to fall ill from the mosquito-borne virus was a 56-year-old Antioch woman. She reported that she started experiencing symptoms Aug. 10.
A 7-year-old Brentwood girl was the second person to start showing symptoms, on Aug. 23. Two days later, a 43-year-old Oakley woman began feeling the effects of the virus.
The most recent West Nile virus victim was a 28-year-old Brentwood woman who started experiencing symptoms Sept. 3.
The county health department is not releasing information about the severity of the illnesses but reported that all four patients are recovering, Contra Costa Health Services spokeswoman Kate Fowlie said.
Contra Costa Mosquito and Vector Control District spokeswoman Deborah Bass said scientists believe many more people are infected with the virus than are diagnosed.
She also stressed that "anybody of any age can get this disease. It's not just the elderly who get sick."
She said that even in its milder form, West Nile virus can cause West Nile fever, which is often accompanied by flu-like symptoms that can last from a few days to a few months and can result in extended hospitalization and even partial paralysis.
More severe forms of the illness can be fatal.
Antioch and Brentwood continue to be the highest-risk areas for contracting West Nile virus, Bass said, and people are urged to continue to wear insect repellent and eliminate backyard sources of standing water.
The mosquito district is also reporting that a horse has tested positive for the virus, the county's first such case this year. The horse, from the unincorporated Byron area, is expected to survive, Bass said.