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.

The measles outbreak in Texas has grown to 124 known cases across nine counties, according to new data released Tuesday afternoon by the Texas Department of State Health Services.

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