A child who tested positive for measles died in West Texas, the state health department said, marking the first death in an outbreak that has sickened nearly 140 people. The Texas Department of State Health Services said a school-age child died after being hospitalized in Lubbock.

The measles outbreak in Texas has escalated sharply to 90 cases as health officials worry about the spread to more communities.

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