Of those infected across the U.S., 97% of cases came from those who are unvaccinated or whose vaccination status is unknown.

The Texas health department reported 646 cases of measles in the state on Friday, an increase of 22 cases from three days ago, as the United States battles one of its worst outbreaks of the childhood disease.

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.