Climate change may worsen the spread of the West Nile virus. What to know. The West Nile virus is more commonly contracted in warmer climates. There, 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, which runs from the summer through the 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.

What is the West Nile virus, and what are the symptoms?

The virus, which mainly spreads to humans through the bite of infected mosquitoes, is maintained in nature in a cycle involving transmission between birds and mosquitoes, the World Health Organization says. It is not contagious between people.

While most patients do not experience symptoms, some may develop headaches, joint pains, rashes, vomiting, or diarrhea, according to the Centers for Disease Control and Prevention. In these cases, most patients have a full recovery, although fatigue may last weeks or months.

The West Nile virus is a human pathogen related to similar viruses, including dengue, zika and yellow fever. About 80 percent of infections in humans are asymptomatic, health authorities say. A small number of people may develop severe illness affecting the central nervous system, such as encephalitis — inflammation of the brain — or meningitis — inflammation of the membranes that surround the brain and spinal cord, which can be fatal in rare cases, the CDC says.

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