Measles Outbreak in Cook County, Illinois, September-October 2025.

AbstractA small community outbreak of measles in suburban Cook County, Illinois occurred in September-October 2025. Cook County Department of Public Health investigators identified four measles cases, including one unvaccinated pediatric index case, two vaccinated adult secondary cases, and an unvaccinated pediatric tertiary case. At least 99 individuals were exposed to measles, including 79 (80%) people exposed in emergency department waiting rooms on two separate dates. Incomplete exposure information was ascertained by public health investigators, which might have contributed to preventable health care exposures.