A measles outbreak in Texas’ South Plains neared 100 cases Friday, according to the Texas Department of State Health Services, which warned more cases of the highly contagious virus are likely to pop up in outbreak areas.

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

Measles is a respiratory virus that can survive in the air for up to two hours. Up to 9 out of 10 people who are susceptible will get the virus if exposed, according to the U.S. Centers for Disease Control and Prevention.