WELD COUNTY, Colo. — Health officials in Weld County said mosquitoes in two areas of the county tested positive for West Nile Virus. The two zones that tested positive are regularly tested, the Weld County Public Health Department said.

The areas where the mosquitoes were caught were in the Johnstown, Milliken and Platteville zone, and the other zone was in the Firestone, Dacono and Fort Lupton area, officials said. Mosquitoes caught in the Greeley, Evans, Kersey and LaSalle zone did not test positive for the virus. > Video above: 9NEWS health expert Dr.

Payal Kohli talks about the growing concern of West Nile Virus infections. Weld health officials said that while no human cases of West Nile have been reported, populations of the mosquitoes that carry the virus are rising. “The increasing hot weather often leads to favorable conditions for the Culex mosquito that carries West Nile virus,” Jason Chessher, Executive Director of the Weld County Health Department, said.

“The public needs to be vigilant, because West Nile Virus has a permanent summer presence in Colorado.” West Nile Virus symptoms usually appear three days to two weeks after a person has been infected. Symptoms can include: Fever Headache Nausea and vomiting Muscle aches and weakness Rash If people develop symptoms, they're urged to seek healthcare immediately, the Weld County Public Health Department said.

Officials said most people infected don't show any symptoms. Less than 1% of those infected devel.