Health officials announced Monday that Colorado’s second case of measles in 2025 was confirmed in an infant in Denver — and warned people who visited the Denver Health emergency room on Sunday, April 6 that they might have been exposed.

The confirmation came one week after officials announced a separate measles case in Pueblo, marking the first time in at least 12 years that the state has confirmed two active measles cases at the same time.

The infections come amid an alarming rise in measles outbreaks nationwide, including several clusters in west Texas and neighboring states that have caused at least two deaths of unvaccinated children. Falling rates of vaccination, fueled by anti-vaccine conspiracy theories like those spread by newly confirmed Health and Human Services Secretary Robert F. Kennedy Jr., have preceded many of those outbreaks.

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In a press release, the Colorado Department of Public Health and Environment said the Denver case was confirmed in an unvaccinated Denver infant under one year old, who had recently traveled to Chihuahua, Mexico. Children are recommended to receive their first dose of the combined measles, mumps and rubella, or MMR, vaccine at between 12 and 15 months old.

“Infants under 12 months are especially vulnerable to measles because they are typically too young to be vaccinated,” Dr. Rachel Herlihy, Colorado’s deputy chief medical officer and state epidemiologist, said in a statement. “This case is a stark reminder that families traveling internationally should delay unnecessary travel or talk to their health care providers about early MMR vaccination for infants, especially when visiting areas with known measles outbreaks.”

The World Health Organization has linked several reported measles cases in Mexico to the Texas outbreak. The Pueblo resident infected with measles last month, an unvaccinated adult, had also recently traveled to Mexico, CDPHE officials said.

Measles is a highly contagious and often severe disease that causes symptoms including fever, cough and a characteristic rash. Cases in the U.S. plummeted following the introduction of a vaccine in the 1960s, and the recommended two doses of the MMR vaccine provide about 97% protection against infection.

Colorado had the nation’s sixth-lowest rate of MMR vaccine coverage for kindergarteners in the 2023-24 school year, with an estimated only 88.3% of kindergarten students vaccinated, according to federal data. While state law requires K-12 students to receive the MMR vaccine and five others, it allows for broad exemptions on medical or religious grounds.

People who visited the Denver Health Emergency Department between 10:30 a.m. and 4:30 p.m. on April 6 may have been exposed to measles, CDPHE officials said. People who were exposed should monitor for symptoms for at least three weeks after exposure and avoid public gatherings, especially if they are unvaccinated.