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.

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

The effectiveness of the vaccine, coupled with rising vaccination rates, led to such low rates of measles that it was declared eliminated in the U.S. in 2000. In recent years, though, the virus has made a resurgence, as childhood vaccination rates have dropped nationwide.