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.

West Texas measles outbreak reaches 663 cases, with most patients unvaccinated or unknown vaccination status, health officials report.

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.