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

Montana is the ninth U.S. state to have an active measles outbreak. The U.S. was up to 800 cases of measles nationwide on Friday. Texas is driving the high numbers, with an outbreak centered in West Texas that started nearly three months ago and is up to 597 cases.

Texas public health officials on Friday reported an additional 22 measles cases in the ongoing outbreak, for a total of 646 outbreak cases. The state’s Department of State Health Services estimates that one percent of the total cases — or fewer than 10 cases — are actively infectious.