In the last three weeks, 48 cases of measles have been identified, sending 13 patients to the hospital, according to the Texas Department of State Health Services.

Measles is marked by high fevers and a telltale rash. People can spread the infection from four days before the rash develops to four days afterward. In addition to the miserable illness, measles can cause complications, particularly in children younger than 5, pregnant people, adults over the age of 20, and people with compromised immune systems.

Nine of those infected were hospitalized and more cases are likely, health authorities said. U.S. vaccination rates for measles are falling, according to the CDC.