A Texas measles outbreak has doubled in the last week to 124 cases, state health officials said Tuesday, a day after they warned a person infected with the highly contagious disease had traveled from the outbreak zone.

State medical experts have confirmed 124 cases of measles in Texas. This is the largest measles outbreak in Texas in 30 years. Here is how it started and an overview of the situation, as of Feb. 25.

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