West Nile virus is the most common mosquito-carried disease in the U.S., and days with temperatures that allow for transmission are becoming more frequent as the global climate warms, according to a new study from Climate Central. 

The disease spreads easiest when temperatures are warm but not too hot, and as summer's warmth starts earlier and lasts longer, mosquitoes come out sooner and last longer, and many areas end up with more days in the sweet spot for disease transmission.

Climate Central set out to find out how the average number of days with temperatures in that peak transmission range, known as “mosquito disease days,” has changed for 239 cities across the U.S. from 1970 to 2025.

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Ultimately, 95% of those cities have more disease days on average each year, averaging 18 more days each year.

The cities most impacted are San Francisco and Reno, Nevada, which are experiencing an average of 52 and 50 more mosquito disease days each year, respectively.

Only 12 of the cities examined in the study are not experiencing an increase in mosquito disease days, including Austin, Las Vegas and Phoenix.

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On the regional scale, the Southeast, Northeast and Northwest have experienced the largest average increases in annual mosquito disease days since 1970 (20 more days in each region).

Overall, the Southwest and south-central U.S. have the most days with temperatures in the right spot while the northern tier sees the fewest days.

The transmission sweet spot

The ideal temperature range for West Nile virus transmission is estimated at about 73 to 79 degrees, but there is some variation depending on what species of mosquito is transmitting the disease and other environmental factors.

The trick is keeping the extrinsic incubation period, or how long it takes for the virus to replicate within the mosquito so the mosquito is infectious, shorter than the lifespan of the mosquito, since warmer temperatures cause mosquitoes to die quicker.

(MORE: Google wants to release 32 million mosquitoes in the US)

For this particular study, the researchers used temperature ranges specific to each city that ranged from as low as 53.8 to as high as 94.8 degrees.

How to protect yourself

It’s hard to avoid mosquitoes all together, but there are some ways you can minimize the risk of exposure to a mosquito transmitting West Nile virus and other mosquito-borne diseases.

Always wear insect repellent when you go outside. Repellents with DEET perform best, but OLE (oil of lemon eucalyptus) is a good plant-based repellent ingredient.

Wear long sleeves, long pants and socks if you have to be outside during peak mosquito hours (dawn and dusk) to prevent mosquitoes from landing on your skin.

Remove any standing water from your property that mosquitoes could use for breeding.

Some plants are natural mosquito repellents, including:

• Lemongrass
• Lemon Eucalyptus
• Lavender
• Mint
• Rosemary
• Marigolds
• Citronella
• Basil
• Sage

Using a fan outdoors can also help blow mosquitoes away from you, keeping your porch cool and more mosquito-free.

Most people infected with West Nile virus (80%) end up not experiencing any symptoms, according to the CDC, but the other 20% can develop flulike symptoms, like a fever with a headache, body aches, joint pains, vomiting, diarrhea or rash.

While most people with mild illness due to West Nile virus recover completely, fatigue and weakness can last weeks or months.

(MORE: West Nile virus attacks the nervous system as cases surge)

Less than 1% of people infected with West Nile virus experience severe symptoms, but the disease can impact the central nervous system and become deadly. Seek medical help if you experience high fever, neck stiffness, muscle weakness, confusion or tremors.

Sara Tonks is a content meteorologist with weather.com and has a bachelor’s and a master’s degree from Georgia Tech in Earth and Atmospheric Sciences, along with a master’s degree from Unity Environmental University in Marine Science.