The city is warning people to take precautions after mosquitos caught in a trap have tested positive for West Nile virus.

It's the first positive test for the virus in the city this year, the city said in a press release. The trap was located in lower central Hamilton.

"The current risk for Hamilton has moved from low to moderate based on receiving these positive results," the release reads.

Most people, around 80 per cent, don't develop any symptoms from West Nile if contracted. But the other 20 percent, especially older adults or those with weakened immune systems, may get West Nile fever. In about 1 per cent of cases, people may develop "severe illness including inflammation of the brain or the lining of the brain."

If symptoms are going to show up, they'll appear between two days and two weeks after a person is bitten by an infected mosquito.

The city says it's treating catch basins and surface water on public land in an effort to control the spread of the virus.

Public health is also advising that people use bug spray that uses DEET, as well as covering up with light coloured long sleeves and pants when in wooded areas, or places where mosquitos are often seen.