The Massachusetts Department of Public Health has raised the West Nile Virus risk level from low to moderate in Wellesley after mosquito samples here and in neighboring communities tested positive. The virus, usually transmitted to humans via mosquito bites, can cause serious illness.

Three human cases of West Nile Virus have been reported in the state, though none in Wellesley. The Wellesley samples were collected on Aug. 31.

The mosquitos that carry the virus have thrived in the damp and humid conditions locally this summer.

While Wellesley does not regularly spray for mosquitoes, the state’s Department of Public Health and the East Middlesex Mosquito Control Commission will add surveillance activities and larvicide applications here and in other moderate-level communities.