Despite a declaration that measles was fundamentally eliminated in the United States in 2000, Idaho health officials are warning travelers of a possible measles exposure at Boise Airport as neighboring Utah becomes the nation's most active measles outbreak epicenter.

A person infected with measles traveled through Boise Airport between 1:30 a.m. and 7:40 a.m. on Sunday, March 29, the Idaho Department of Health and Welfare announced. Local public health districts have notified airport officials and are contacting travelers who shared flights with the infected individual.

Measles is highly contagious. The virus spreads through the air when an infected person coughs, sneezes or talks, and unprotected individuals can contract it up to two hours after an infected person has left a room or enclosed area. Early symptoms include fever, runny nose, cough and red, watery eyes, followed by a rash. In severe cases, measles can cause pneumonia, brain infection and death.

The Boise exposure comes as Utah's outbreak has become the most active in the country. With more than 121 cases reported in the previous three weeks — and South Carolina recording no new cases since March 17 — the University of Minnesota's Center for Infectious Disease Research and Policy identified Utah as the current epicenter. Utah epidemiologist Leisha Nolen said the state has not seen case counts this high in more than 40 years.

Of Utah's 583 confirmed cases, 386 have been diagnosed in 2026 and 197 in 2025, according to the Utah Department of Health and Human Services. The state is recording roughly 35 new cases per week. Forty-seven people have been hospitalized. About 383 of those infected are under 18. Approximately 485 — more than 83 percent — were unvaccinated at the time of diagnosis.

The outbreak is concentrated in southwestern Utah. The Southwest Utah Public Health Department, which covers Washington, Iron, Kane, Beaver and Garfield counties, has recorded 249 cases. Utah County has seen 93 cases, Salt Lake County 62, the Tri-County region of Daggett, Duchesne and Uintah counties 59, and the Central Utah Public Health Department's service area 50. Infections that began in the southwest spread to other parts of the state in February.

During the 2024-25 school year, roughly 10 percent of Utah's in-person kindergarten students had an exemption for the measles, mumps and rubella vaccine or were missing documentation.

Measles was declared eliminated in the United States in 2000, a status achieved through high vaccination rates, though imported cases continue to cause sporadic outbreaks. Many countries — including popular tourist destinations — are currently experiencing outbreaks, and the risk of travel-related exposure remains ongoing.

The MMR vaccine is highly effective at preventing measles and is routinely recommended for children 12 to 15 months of age, with a second dose at 4 to 6 years. Infants younger than 12 months are not routinely vaccinated, though those between 6 and 11 months are advised to receive an early dose before international travel.

Given the scale of Utah's outbreak, state health officials are now recommending that early dose for all Utah infants 6 to 12 months old, regardless of travel plans. Infants who receive an early dose will still need the two standard doses at 12 to 15 months and 4 to 6 years.

Symptoms typically appear seven to 14 days after exposure. Health officials urge anyone who develops symptoms — particularly high fever of 101 degrees Fahrenheit or higher, cough, runny nose or red eyes — to avoid contact with others and call a health care provider before visiting a clinic or hospital. Anyone unsure of their vaccination status is encouraged to consult a doctor.

For more information about measles, visit cdc.gov/measles/about. For international travel health notices from the Centers for Disease Control and Prevention, visit cdc.gov/travel/notices.