The 2017 West Nile season has begun.

The Larimer County Department of Health and Environment said Thursday morning that mosquitoes infected with the virus were trapped in Berthoud last week – the first positive samples of the year.

The West Nile virus is most frequently transmitted in July and August, as the weather gets hotter.

Colorado has had human cases of the West Nile virus every year since 2002. People who work outside or participate in outdoor activities are at the most risk.

The symptoms of West Nile start two to 14 days after infection. In rare cases, the virus can cause a serious brain infection.

Health officials say you should wear mosquito repellents when you go outside in at-risk areas, as well as protective clothing.

Read more about the virus and how to protect yourself here: