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.

Texas health officials reported 17 new measles cases on Tuesday, bringing the total confirmed cases since late January to 663. The state’s public health department estimates that fewer than 10 of the confirmed cases — about 1% — are “actively infectious.

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.