The death is linked to an outbreak concentrated in three counties in southwestern Wisconsin: Iowa, Grant and Lafayette. The coroner listed measles as the immediate cause of death.

"The important message is that community levels of vaccination are important to protect those that are too young to be vaccinated," said Dr. Ryan Westergaard, chief medical officer for the state health department and state epidemiologist for communicable diseases.

Measles is a highly contagious airborne virus that can linger in the air for up to two hours after an infected person coughs, sneezes, sings or breathes. It can cause pneumonia, brain swelling and death.

Wisconsin has confirmed 431 measles cases this year, including more than 400 since August, according to state data. More than 99% of this year's cases were in unvaccinated people, and nine residents have required hospitalization. By comparison, the state recorded 36 cases in 2025.

"For perspective, the last time we had even more than 50 cases in a year was more than 35 years ago," Westergaard said. "The interesting historical point is that the last resurgence of measles in this country in 1991 was the impetus for the recommendation to get two doses of MMR instead of one."

The measles, mumps and rubella vaccine is about 97% effective with two doses. The first dose is recommended for children 12 to 15 months old, with the second at age 4 to 6. Infants as young as 6 months can receive an early dose if they are traveling or live in an outbreak area. Since August, between 500 and 600 infants in Wisconsin have received an early dose, according to Dr. Stephanie Schauer, the state's immunization program manager.

Wisconsin's average childhood MMR vaccination rate is 85%, below the 95% threshold needed for broad community protection. In Grant County, which has seen 142 confirmed or probable cases, the vaccination rate for school-aged children is 67%. Dane County's rate for students ages 6 to 18 was 83% as of 2025.

All 13 counties that have reported measles cases this year have average vaccination rates below 95%, according to the health department. The counties and their case counts as of Thursday are: Buffalo (1), Chippewa (44), Dane (1), Grant (142), Iowa (97), Jackson (2), Lafayette (115), Marathon (6), Marquette (2), Richland (1), Rock (5), Vernon (14) and Waukesha (1).

Dr. Jim Conway, a pediatric infectious disease specialist with UW Health Kids in Madison, said the state's slipping vaccination coverage made an outbreak predictable.

"We've been kind of a sitting duck, to be honest, for a long time," Conway said. "Because it's such a contagious disease, it doesn't take much for an outbreak to get started. And once an outbreak starts with measles, it's really, really hard to shut it down."

Conway said he and colleagues modeled likely outbreak locations and identified Texas, Pennsylvania and Wisconsin based on declining immunization rates.

"Measles, in some ways, is sort of the canary in the coal mine," he said. "What I worry is that it could be the tip of the iceberg, too."

Westergaard noted that vaccine uptake has fallen since the COVID-19 pandemic and that public health officials have tracked misinformation about vaccines, including false claims linking vaccines to autism repeatedly spread by federal Health and Human Services Secretary Robert F. Kennedy Jr.

"I think it is fair to say there has been some public shift in the attitudes and beliefs around vaccination," Westergaard said. "But there have not been changes in the evidence that allows medical professionals and public health agencies to be confident in the vaccination."

A report released Thursday by the Wisconsin Policy Forum found that the share of K-12 students meeting the state's minimum immunization requirements remains near a record low. The state's childhood vaccination rate peaked at 97.1% in 1986 and has declined since.

The United States achieved measles elimination status in 2000, meaning cases were minimal and typically linked to travel, with little person-to-person spread. Westergaard said that achievement was not permanent.

"Measles was once a common childhood infection in the United States. What brought it under control was vaccination to the point that many physicians practicing today have never seen a case," he said. "That was an extraordinary public health achievement, but it wasn't permanent."