They can carry the West Nile Virus, which while uncommon, can lead to complications.
Some people may not experience symptoms, but the typical ones are flu-like.
Somtimes the virus can cause severe illness such as meningitius, especially in older people.
Ways to protect against the virus are using repellant with DEET, wearing longer clothing and getting rid of standing water.
For more information on how to protect yourself, call your local Health Unit office.
Meantime, public health officials in Toronto and St. Catharines are reporting finding mosquitoes carrying West Nile virus.
Toronto Public Health says it received a laboratory report indicating that mosquitoes have tested positive for West Nile.
Toronto associate medical health officer Dr. Howard Shapiro says the finding of mosquitoes carrying West Nile is earlier in the season than previous years.
In St. Catharines, environmental health manager Peter Jekel says all Niagara communities should be on alert.
No birds have tested positive for West Nile so far this summer in Niagara and the region has not seen any human cases.
Halton Region Public Health last week reported finding West Nile in two mosquito ponds.
