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.

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.

The Texas Department of State Health Services reported Tuesday its measles cases, which date back to late January, had grown to 663 cases as of Tuesday, marking a jump of 17 cases since the state’s last update as 87 of the patients have been hospitalized while two school-aged children have been confirmed dead from the illness.