At least 124 cases have been reported since late January, mostly among children and teenagers who were unvaccinated or whose vaccination status was unknown.

According to the Texas Department of State Health Services (DSHS), the victim was an unvaccinated school-aged child who died on Feb. 26 in a hospital in Lubbock after being diagnosed with measles last week.

Here’s what to know about the outbreak after the first U.S. death since 2015 from the highly contagious—but preventable—respiratory disease.