It's the latest outbreak of a disease that had been virtually eliminated in the U.S., and it comes as vaccination rates are declining — jeopardizing the country's herd immunity from widespread outbreaks.

A measles outbreak is growing in a rural area of West Texas where vaccination rates are well below the recommended level.

The Texas Department of State Health Services has issued guidance to schools as the measles outbreak in Gaines County rises to 15 cases, mostly in children.

Public health practitioners warn such outbreaks will become more common because of scores of laws around the U.S. that ultimately lower vaccine rates.

On January 30, officials reported two measles cases in unvaccinated, school-age children in Gaines County, which sits at the border of New Mexico and is around 90 miles southwest of Lubbock, Texas. Both children were hospitalized in Lubbock and had been discharged.