Search Climate Insights Directory

Climate Central's analysis of 239 major U.S. cities from 1970 to 2025 found that 95% of these cities experienced an increase in 'mosquito disease days'—days with temperatures suitable for West Nile virus transmission. On average, these cities saw an increase of 18 days per year. The largest increases occurred in the Southeast, Northeast, and Northwest regions, with San Francisco, CA, seeing the highest increase of 52 days. The report also notes that while West Nile virus is the most common mosquito-borne disease in the U.S., warming is expanding the risk and range of other viruses like dengue and Zika, particularly along the Gulf Coast and into the Ohio Valley and Upper Midwest by 2050.

Climate Central analyzed temperature data from 244 U.S. cities to determine the increase in days falling within the 61-93°F range suitable for disease transmission by Aedes and Culex mosquitoes. The analysis found that 94% of these cities are experiencing an increase in these 'disease danger days' since 1970, with significant increases noted in California, the Southwest, and some northern cities. While most areas face higher risks, some locations like Phoenix may become too hot for mosquito survival. The report also highlights that warmer temperatures shorten the Extrinsic Incubation Period (EIP) of viruses, potentially increasing the number of infectious mosquitoes and expanding the range of species like Aedes albopictus.