Colorado is experiencing an unprecedented outbreak of West Nile virus. West Nile virus (WNV) is the leading cause of mosquito-borne disease in the continental United States.

A Weld County resident recently died from WNV, marking the state’s first West Nile-related fatality in 2023, officials announced this month. It’s unlikely to be the last WNV death considering there is an “abundance” of Culex mosquitoes, the primary species that carries the virus in the U.S., swarming due to record rainfall this past winter and spring.

“The trends we are seeing in our West Nile virus tracking data are unprecedented,” Colorado epidemiologist Rachel Herlihy said in a statement Friday according to a report by Axios. “This is especially concerning now that August is here and September is just around the corner, as this is usually when human cases peak in Colorado,” Herlihy added.

Twenty people died from West Nile in Colorado last year, and more than 200 were infected.

According to the Centers for Disease Control and Prevention’s website, WMV is most commonly spread to people by the bite of an infected mosquito. Cases of WNV occur during mosquito season, which starts in the summer and continues through fall. There are no vaccines to prevent or medications to treat WNV in people. Fortunately, most people infected with WNV do not feel sick. About 1 in 5 people who are infected develop a fever and other symptoms. About 1 out of 150 infected people develop a serious, sometimes fatal, illness.

Severe illness can occur in people of any age; however, people over 60 years of age are at greater risk for severe illness if they are infected (1 in 50 people). People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk.

Symptoms of severe illness include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis.