A child who wasn't vaccinated died in a measles outbreak in rural West Texas, state officials said Wednesday, the first U.S. death from the highly contagious — but preventable — respiratory disease since 2015.

An unvaccinated child has died as the Texas measles outbreak reaches 124 cases. Public health officials also warn of potential spread in San Marcos and San Antonio.

The measles outbreak in West Texas has risen to 124 cases across nine counties, the worst outbreak in the state in over 30 years. The majority of the cases center on the Mennonite communities, where many of the children are “undervaccinated,” state health department spokesperson Lara Anton said.