At least 124 cases of measles have been reported in Texas since late January — marking the largest outbreak in the state in close to 30 years. The dozens of reported cases have mostly affected children,

Health authorities warned of further spread. Most U.S. measles cases this year involved people 19 and under and those without a confirmed vaccination.

Texas is in the midst of the worst measles outbreak in decades, with 90 cases across 7 counties. Here’s what to know.