A batch of mosquitoes captured in Hamilton this week have tested positive for West Nile virus.

They're the first group of the pests to be found with the virus this year, according to a media release from the city, which announced the medical officer of health has moved the risk level from low to moderate.

"It's important to take precautions in the summer months to avoid illnesses like West Nile Virus and Lyme disease, which are spread by biting insects," said Associate Medical Officer of Health Ninh Tran.

"If you plan to be outdoors, using an insect repellent containing DEET or Icaridin, and wearing loose fitting, light-coloured clothing will help reduce your risk of being infected."

The city is also reminding residents to remove all standing water to keep mosquitoes from breeding.

While about 80 percent of people infected with West Nile show no symptoms, older adults or those with weakened immune systems may get a fever or suffer more severe illnesses such as inflammation of the brain, reads the release.

People are encouraged to protect themselves by covering up, using bug spray and avoiding areas where mosquitoes are known to gather, especially around dawn and dusk.