Aug. 20 was World Mosquito Day, marking the 1897 discovery that mosquitoes spread disease. More than a century later, there's still no vaccine for West Nile — and the virus is spreading faster in the U.S. than it has in more than 20 years, partially due to weather.

The pace of spread is striking. By July 28, the count stood at 113 cases, roughly 2.5 times the number reported at the same point in 2003—the worst West Nile year on record, which ultimately produced 9,862 cases and 264 deaths.

As of Aug. 18, the ArboNET data from the CDC has confirmed 222 cases across 32 states — up from 181 on Aug. 11 — and at least eight deaths, five in Arizona and three in California.

Most of the current cases are severe. 156 are neuroinvasive, meaning the virus reached the brain or spinal cord. That means hospitalization, and for some patients, permanent damage.

And the season hasn't peaked. Transmission typically peaks from late August through September. The worst weeks are still ahead. 

The weather built this outbreak

It started with a milder winter. Fewer mosquitoes died off, which left room for the population to grow once temperatures climbed

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Heat is the accelerant: Warm weather speeds up both mosquito development and the growth of the virus inside the insects, so populations build faster and turn infectious sooner. Arizona, Texas and California have run relentlessly hot all summer, and those three states lead the case count.

Cities compound it: The urban heat island keeps metro Phoenix warm long after sunset, and warmer nights mean faster development — part of why 76 of Arizona's cases cluster there rather than in the rural parts of the state.

The breeding ground: In Arizona, monsoon rain fell on ground too baked to absorb it and pooled in storm drains, retention basins and washes; in Texas, the flooding that hit in July left standing water in ditches, fields and low ground; in California, the heat came early and the water was already there.

The flood makes news, then the story moves on but what's left behind is prime habitat for Culex, the mosquito that carries West Nile, for as long as it takes to evaporate. In a hot summer, that's weeks.

How to protect yourself

The heat also changes what we do. Triple-digit afternoons push everything outdoors into the cooler hours — the evening walk, the patio dinner, the early run. That's the same window mosquitoes bite hardest.

As Johns Hopkins puts it: "Temperatures warm up, bird populations build up, and mosquito populations build up, and then people are outside more — having picnics, sitting on their decks. That's when exposure happens."

With peak transmission still ahead, here are five things you can do before you step outside.

Avoid dusk and dawn. Cooler temperatures for us, but mosquitoes are often most active around sunrise and sunset, so limit outdoor time during these hours when possible.

Use an EPA-registered repellent. Apply a registered insect repellent to exposed skin and follow label instructions.

Wear light layers. Choose lightweight, long sleeves and pants to reduce exposed skin without overheating.

Get rid of standing water. Empty water from buckets, planters, toys, and other containers where mosquitoes can breed. Clear storm drains when you can.

Know the signs. Pay attention to symptoms such as fever, headache, body aches, or rash after mosquito exposure.

Faiza Mohammed is a meteorologist intern at weather.com. She studied geography and worked with the Weather Dawgs team at the University of Georgia. She also received her master's in Earth and Atmospheric Sciences from the Georgia Institute of Technology.