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.

Confirmed measles cases in Texas have reached at least 90, making this the worst measles outbreak in 30 years. Experts warn the number of Texas cases is likely much higher.

The Hays County Health Department has issued an alert of a potential measles exposure in San Marcos. The San Antonio Metropolitan Health District has issued an alert of a potential measles exposure there.