The outbreak would have to spread continuously for more than a year to undo the U.S.’s achievement.

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.

The number of cases reported in Texas' historic measles outbreak has risen to 663 cases in 26 counties, as of April 29. Of those, 87 patients have been hospitalized, and two school-aged children have died since the outbreak began in January.