Texas' measles outbreak is up to 90 cases. Here's what you need to know about the virus, vaccination and boosters.

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.

The largest measles outbreak in decades has reached San Antonio and San Marcos. Officials say an individual who tested positive for the virus in West Texas traveled to two major universities and one of the nation's busiest tourist attractions — the San Antonio River Walk.