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.

With one-fifth of states seeing active measles outbreaks, the U.S. is nearing 900 cases, according to figures posted Friday by the U.S. Centers for Disease Control and Prevention. The CDC’s confirmed measles case count is 884,

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.