The U.S. declared measles eliminated 25 years ago. But the growing outbreak of the disease centered in West Texas poses a threat to this coveted status—and signals the possibility of measles becoming more common.

The number of cases reported in Texas' historic measles outbreak has risen to 663 cases in 26 counties, as of April 29. Of those, 87 patients have been hospitalized, and two school-aged children have died since the outbreak began in January.

As more counties are being impacted by the ongoing measles outbreak and added to the growing list of counties across Texas, here's what to know.