Officials say further tests are needed to confirm the infection.
There were no details on the person's age or gender.
The Middlesex-London Health Unit says one mosquito trap and three dead crows found in the region have tested positive for West Nile.
Earlier this month, health officials confirmed a Toronto man had contracted the infection in July.
West Nile is transmitted to humans mainly through mosquito bites, but health officials say the risk of being infected is low and most people who are don't get sick.