The Centers for Disease Control and Prevention has counted 504 human West Nile virus cases across 41 states this year, and 363 of them, about 72 percent, are neuroinvasive, the severe form in which the virus affects the brain or spinal cord. Those figures come from CDC's ArboNET surveillance data current as of Sept. 8, as summarized by the Annenberg Public Policy Center. CDC refreshed its dashboard again on Sept. 15, and counts are expected to keep rising because case reports lag behind illness.

The calendar is the reason to pay attention now. West Nile transmission typically peaks from late August through September, and CDC says the mosquitoes that spread the virus are most active between dusk and dawn, when many people walk dogs, coach practices or eat outside. There is no vaccine or specific medicine to treat West Nile virus in people.

The season's shape has changed since early summer. CDC reported the earliest start to a West Nile season in the United States, with more cases by the end of June than at that point in any year since 2004. By Sept. 8, however, the national count was well below the 771 cases CDC had reported by the same point in 2025, according to last year's count at the same point. That makes 2026 a story about severe illness more than record case totals.

An Early Surge Slowed, but Severe Cases Kept Coming

CDC had recorded at least 48 cases by June 30, 38 of them neuroinvasive, compared with an average of about 10 cases by the end of June since 2004, and 23 states were reporting West Nile activity, the most at that point in a decade, the agency said in its early July statement. "These findings serve as an important reminder that mosquito season is well underway," CDC medical epidemiologist Dr. Erin Staples said at the time.

By Sept. 1, CDC listed 407 cases in 39 states and Washington, D.C., including 285 neuroinvasive cases and 23 deaths, AccuWeather and Fox News reported. The Sept. 8 update added 97 cases.

The high severe share needs context. CDC notes that mild illness is far more likely to go unreported than neuroinvasive disease, so the 72 percent figure describes reported cases, not all infections. Even so, the share is higher than last year, when 490 of the 771 cases reported by early September, about 64 percent, were neuroinvasive. Most infected people never develop symptoms; about 1 in 5 develop a fever and other symptoms, and fewer than 1 percent develop severe neurologic disease. CDC counts cases by county of residence rather than where a bite occurred, and its data are preliminary.

California and Illinois Report Deaths

State data often run ahead of CDC's. California's West Nile virus tracking site listed 86 human cases in 21 counties and six deaths as of Sept. 4, after 22 new cases in a single reporting week. The deaths were reported from Butte (two), Glenn, Kings, Los Angeles and Sacramento counties, and the California Department of Public Health says virus activity in the state is at a five-year high. In Illinois, Chicago health officials confirmed the city's first West Nile death of the year in early September.

Arizona was the early epicenter. Maricopa County, home to Phoenix, Mesa and Scottsdale, reported 29 cases and five deaths as of July 1, compared with 24 cases and two deaths on the same date in 2025.

Weather helped set the stage. NOAA reported that summer 2026 was the warmest on record for the Lower 48, and The Weather Channel noted that a milder winter let more mosquitoes survive into the season. Warm conditions generally speed mosquito development and the growth of the virus inside mosquitoes, though no published analysis has yet measured how much this year's heat shaped case totals.

Adults Over 60 Face the Greatest Danger

Severe illness is concentrated, not universal. CDC says adults 60 and older and people with certain medical conditions, such as cancer, diabetes, high blood pressure, kidney disease or an organ transplant, face the highest risk of severe disease. Outdoor workers, evening exercisers and residents of neighborhoods with standing water or neglected pools have more exposure.

Symptoms typically appear within two weeks of a bite and can include fever, headache, body aches, joint pain, vomiting, diarrhea, or rash. A severe headache, stiff neck, confusion, vision loss, muscle weakness, or seizures after mosquito exposure needs emergency care. West Nile virus is not spread through casual contact such as touching or coughing.

Knowing the risk is not the same as acting on it. The Annenberg survey of 1,904 U.S. adults, fielded Aug. 4 to 17, found that 82 percent know preventing bites and controlling mosquitoes is the best defense, and 64 percent say they routinely take precautions. But only 31 percent knew CDC advises applying sunscreen first, letting it dry, then applying repellent; just 15 percent knew repellent should not go on skin under clothing; and only 18 percent knew to look for an EPA-registered product.

"Despite the drop in the number of West Nile Virus cases relative to 2025, taking precautions to prevent mosquito bites and control mosquitoes is important to uphold community public health," said Ken Winneg, the center's managing director of survey research.

Repellent Habits and Standing Water Before the First Freeze

The protective steps are inexpensive. CDC's prevention guidance points to EPA-registered repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol or 2-undecanone, and CDC notes that concentrations above 50 percent DEET generally add no extra protection. Do not use insect repellent on babies younger than 2 months; cover strollers and carriers with mosquito netting instead.

Once a week, empty, scrub, or throw out items that hold water, such as flowerpot saucers, buckets, bird baths, tarps, and clogged gutters, since even small amounts can breed mosquitoes. Repair window screens and wear loose long sleeves and pants outdoors between dusk and dawn. Local mosquito and vector control agencies handle problems such as neglected pools, so report concerns rather than treating a neighbor's property yourself.

Canceling outdoor plans is not necessary for most people. Older adults can shift walks and yard work away from dusk and dawn, and anyone with severe symptoms after mosquito bites should seek care promptly and mention the exposure.

CDC updates ArboNET every one to two weeks from June through December, and transmission usually falls after the first sustained cold weather. Because reporting lags illness, case counts will likely keep rising into October even as mosquito activity fades. Nature World News will follow the next CDC and state updates.

The bottom line: National West Nile counts are running below last year, but about 7 in 10 reported cases are severe, and the late-season risk window is still open. Adults over 60 have the most to lose, and consistent repellent use and weekly removal of standing water remain the most effective defenses until the first hard freeze. Check with your county health department for local mosquito activity.

What Readers Want to Know

How many West Nile cases has the U.S. reported in 2026? CDC counted 504 human cases in 41 states as of Sept. 8, including 363 neuroinvasive cases. The data are preliminary and are updated every one to two weeks.

Is 2026 a record West Nile year? Not by case count so far. The season started early, with more cases by the end of June than in any year since 2004, but the Sept. 8 total was well below the 771 cases reported by the same point in 2025.

Why are so many reported cases severe? Severe cases are more likely to be diagnosed and reported, while mild cases are often missed. The 72 percent share reflects reported cases, not all infections.

Who faces the greatest risk? Adults 60 and older and people with conditions such as cancer, diabetes, high blood pressure, kidney disease, or an organ transplant. Outdoor workers and evening exercisers have more exposure.

What symptoms need emergency care? A severe headache, stiff neck, confusion, vision loss, muscle weakness, or seizures after mosquito exposure.

Which repellents work? EPA-registered products with DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undecanone. Apply sunscreen first, let it dry, then apply repellent to exposed skin only.

When does West Nile season end? Transmission usually drops after the first sustained cold weather, but reported cases can keep rising for weeks because of reporting delays.