Colorado health officials are warning people who visited four Boulder County locations last week that they may have been exposed to measles by an international visitor with a confirmed infection.

The exposure windows identified by the Colorado Department of Public Health and Environment were Avanti Food & Beverage Boulder, including the first floor and rooftop, from 7 to 11:30 p.m. September 8; Gelato Boy West End on Pearl Street from 9:30 to 11:45 p.m. that night; Pearl Street Mall from about 9:45 p.m. September 8 through 12:30 a.m. September 9; and Good Samaritan Hospital’s emergency department in Lafayette from 7:30 to 8:30 p.m. September 11. The locations and times were published through the state’s measles exposure notice and reported by Axios Boulder.

People who were at those places during the listed periods should monitor themselves for symptoms for 21 days after the potential exposure. That means the final monitoring dates vary by location, extending into September 29 for the September 8 exposures and through October 2 for the hospital exposure.

Typical symptoms begin with fever, cough, runny nose and red or watery eyes, followed several days later by a rash that usually starts on the face and moves downward. Anyone who develops possible measles symptoms should call a health care provider before arriving in person, allowing the clinic or hospital to take precautions that limit additional exposure.

That warning reflects how easily measles spreads. The Centers for Disease Control and Prevention says the virus can remain infectious in a room for up to two hours after an infected person leaves, and as many as nine in 10 unprotected people who have close contact can become infected. Children younger than five, adults older than 20, pregnant people and those with weakened immune systems face higher risks of serious complications.

Colorado has recorded 25 measles cases in 2026, including seven hospitalizations, according to state data cited in the exposure report. The new case follows a February outbreak centered on Broomfield schools that grew to nine cases and required quarantine and contact tracing.

The warning also comes during a national resurgence. CDC’s September 11 surveillance update counted 3,294 confirmed U.S. cases as of September 10, already above the 2,289 cases reported in all of 2025. Ninety-five percent of this year’s confirmed cases were linked to outbreaks, and Colorado was among the jurisdictions reporting infections.

Vaccination status is the most important practical distinction for people assessing their risk. CDC says two doses of the measles, mumps and rubella vaccine are about 97% effective at preventing measles, while one dose is about 93% effective. People who are unsure whether they are protected should contact a medical provider or check the Colorado Immunization Information System rather than waiting for symptoms. The current notice is an exposure warning, not evidence that transmission occurred at every listed site.