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.

The measles outbreak first reported in Gaines County, Texas, has doubled to 48 cases in the last four days, the Texas Department of State Health Services said in a statement Friday.

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.