As measles cases surge across the U.S., reaching the highest numbers in decades, public health officials are increasingly issuing exposure alerts to help communities respond quickly to prevent further spread.
Why swift action is needed: Measles is highly contagious, with a single person infecting up to 18 other unprotected individuals. And, the virus doesn’t just spread through close contact—it “can remain infectious while suspended in the air, or survive on surfaces, for up to two hours,” says William Moss. That means that someone can be exposed even after an infected person has left the room. In communities where vaccination coverage is below the recommended threshold of 95%, even one case can quickly snowball into a broader outbreak.
What exactly is a measles exposure alert? When someone with a confirmed case of measles has been in a public place while contagious, health departments attempt to contact everyone who may have been exposed. If they are unable to, they issue an alert—often via news releases and social media—with the exact location, date, and timeframe when the infectious individual was present (e.g., “Tuesday, February 10, between 2 p.m. and 6 p.m., at the North Train Station”), so that anyone who was also in that location around that time can act quickly to protect themselves.
These alerts are “precautionary, and not a reason to panic,” says Darcy Phelan-Emrick. Their purpose is to give people clear, time-sensitive information so they can make informed decisions about vaccination, monitoring symptoms, or contacting a provider.
From the Johns Hopkins Bloomberg School of Public Health Wednesday, February 25, 2026