The Simcoe Muskoka District Health Unit says lab results show two probable cases of the virus, with one of those likely being contracted in New Tecumseth.
Region of Waterloo Public Health confirmed Wednesday a local case of West Nile, although a spokesperson said the virus was acquired outside of the region.
Two Guelph area residents have tested positive for West Nile virus.
It brings the number of positive tests from mosquitoes in Perth County to six.
Joe Lynch, program manager for vector control and nuisances for the county board of health, said he couldn't release more details on the affected individual.
About 80% of people who get the virus don't know they have it.
West Nile virus is spread to humans by the bite of an infected mosquito.
Treating standing water with larvicide also can reduce the potential for West Nile virus transmission, according to the news release. When infected, some people will experience flu-like symptoms. During that time, people should wear insect repellent to prevent mosquito bites, the board of health said.
Health officials say the infection rate is moderate this year.
"We had a warm winter, we've had a cool, wet summer, and all that contributes to the possible- kind of flawless storm for West Nile virus in southern Ontario this year".