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 Texas health department reported 561 cases of measles in the state on Tuesday, an increase of 20 from April 11, as the United States battles an outbreak of the childhood disease that began in the West Texas area.