The Centers for Disease Control and Prevention reported 800 confirmed cases of measles in the U.S. as of Friday, making the announcement as the highly contagious illness has reached half of the country—though most cases remain focused in Texas.

The measles outbreak in West Texas now counts almost 600 cases, according to Texas health data released Friday. The 597 cases is 36 more than were tallied Tuesday, according to information from Texas Health and Human Services.

Texas health officials reported 36 new measles cases Friday, bringing the total number of confirmed cases since late January to 597. The state’s public health department estimates that fewer than 30 of the confirmed cases — about 4% — are “actively infectious.