Measles is caused by a highly contagious virus that’s airborne and spreads easily when an infected person breathes, sneezes or coughs. It is preventable through vaccines, and has been considered eliminated from the U.S. since 2000.

New Mexico health officials announced that the measles outbreak has reached a third county in the state’s southeastern corner Tuesday.

Texas and New Mexico measles outbreaks grow, with Texas reporting 481 cases across 19 counties, affecting largely undervaccinated areas.