West Nile virus activity in the United States is running at its fastest start in two decades: the CDC recorded 48 human cases by June 30, 2026, roughly five times the typical count for that point in the season, and counts compiled from federal data reached 143 cases through August 4 as peak transmission weeks began.
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Why 2026 looks different
In an average year, only about 10 human cases are reported nationwide by the end of June, per the CDC; the 48 cases recorded by that date this year spanned 23 states, the earliest and broadest start since 2004, according to agency data cited in coverage of the season. Transmission typically rises through July and peaks in August and September, which means the current totals are likely a floor rather than a ceiling. Case counts tend to spike roughly every three years, and conditions this summer have fit that pattern, per reporting on the season.
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What the case counts can and cannot show
Reported cases skew toward the minority of infections that cause severe illness. Most people infected never develop symptoms, and roughly one in 150 develops serious neuroinvasive disease such as encephalitis or meningitis, per the CDC, so surveillance totals reflect testing intensity as well as true incidence. There is no human vaccine and no specific antiviral treatment.
When to talk to a clinician
Fever, severe headache, neck stiffness, disorientation, or muscle weakness after mosquito exposure warrants prompt medical attention, particularly in adults over 60 or people with weakened immune systems, who face the highest risk of severe disease.
What prevention looks like
Health departments recommend insect repellent with DEET, picaridin, or oil of lemon eucalyptus, long sleeves at dawn and dusk, window screens in repair, and draining standing water where mosquitoes breed.