The Texas health department reported 663 cases of measles in the state on Tuesday, an increase of 17 cases since April 25, as the U.S. battles one of its worst outbreaks of the childhood disease.

Nearly all measles infections seen during the Texas outbreak are among individuals who have not been vaccinated, or whose vaccination status is unknown.

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.