The two people, who live in the eastern part of the county, were likely bitten by mosquitoes carrying the virus in late September, said Contra Costa Mosquito and Vector Control District spokeswoman Deborah Bass.
Bass said the pair are the fourth and fifth human cases of the virus confirmed in the county this year. Of those five cases, three are believed to have contracted the mosquito-borne illness outside of the county, she said.
"It's very important to remember that the majority of West Nile virus cases are not reported," Bass said in a statement. "Based on data from the CDC, we estimate that in Contra Costa County up to 280 people were affected by the virus last year, they just didn't know it or it wasn't reported."
In addition to the recently confirmed human cases, a dead bird found in Alamo also tested positive for the West Nile virus.
Bass noted that birds infected with West Nile virus are the reservoir for the illness, since the mosquitoes bite them and go on to bite people.
Symptoms of the virus include fever, tiredness, head and body aches and swollen lymph glands. The virus can last from just a few days to several weeks and in rare cases, can be fatal.
Any Contra Costa County residents who believe they have been infected with the virus are encouraged to contact Contra Costa Health Services at (888) 959-9911.