The West Nile virus is more commonly contracted in warmer climates, where hot and tropical temperatures allow the mosquitoes that carry it to thrive. Health experts are increasingly concerned that climate change could worsen the spread of the virus in less common places — and even bring it to new ones. The virus, which is the leading cause of mosquito-borne disease in the continental United States, is particularly dangerous during mosquito season, starting in the summer through fall. “The number of West Nile virus-infected mosquitoes we’ve detected this season is the highest we’ve seen in years,” said Colorado’s state epidemiologist, Rachel Herlihy. For most people, the virus causes no serious illness and requires no treatment. However, in about 1 in 150 cases, it can lead to serious infections of the brain and nervous system. Here’s what to know.

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