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,

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.

The outbreak is primarily affecting West Texas, but officials in San Antonio, New Braunfels and San Marcos are monitoring cases after an infected person recently visited the area, according to the Texas Department of State Health Services.