West Nile Virus by County: How to Read the Case Counts
What the numbers are
CDC's ArboNET system collects every human West Nile case state health departments report, by county of residence. From 1999 to 2025 that comes to 62,990 cases across 2,423 counties, 33,206 of them neuroinvasive (meningitis or encephalitis). 1,815 counties have had at least one case since 2016. Most infections cause no symptoms and are never reported, so the true number of infections is far higher.
Counts, not rates
A count grows with population. Here are the counties with the most cases since 1999:
| County | Human cases | Worst year |
|---|---|---|
| Maricopa County, AZ | 3,155 | 2021 (1,476) |
| Los Angeles County, CA | 2,262 | 2004 (306) |
| Cook County, IL | 1,811 | 2002 (635) |
| Larimer County, CO | 1,122 | 2003 (546) |
| Weld County, CO | 957 | 2003 (402) |
| Dallas County, TX | 929 | 2012 (396) |
| Boulder County, CO | 908 | 2003 (421) |
| Harris County, TX | 824 | 2014 (134) |
Several are among the most populous counties in the country. A small rural county with a handful of cases can carry a higher risk per person. By state, the most cases since 2016: CA (2,691), AZ (2,348), CO (1,847), TX (1,781), IL (925), NE (924).
Outbreak years
West Nile comes in waves. A county can go years with nothing and then have its worst year all at once, which is why each county page shows the peak year and the latest year, not only the total. A hot summer speeds up how fast the virus multiplies in mosquitoes and birds.
When the risk is highest
Most cases begin in late summer, when the virus has built up in birds and mosquitoes. Each county page marks its peak window, the months with a mean of 70°F or more. Across counties the warmest month is most often July (2,897 counties) or August (243 counties). CDC says people over 60 and those with weakened immune systems are most likely to become seriously ill. Use an EPA-registered repellent (DEET, picaridin, IR3535 or oil of lemon eucalyptus) at dusk and dawn in those months.
Where to go next
See the range: Hennepin County, MN · Cook County, IL · Harris County, TX · Miami-Dade County, FL.
Where Mosquito Season Never Ends · How the Mosquito Season Dates Are Worked Out