The measles outbreak in Texas has escalated sharply to 90 cases as health officials worry about the spread to more communities.

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.

A rapidly escalating measles outbreak in West Texas has forced some private schools to close their doors. Local health departments, already stretched thin, are prioritizing efforts to contain the highly contagious virus.