A growing measles outbreak in west Texas has infected 48 people, according to official state data released Thursday -- the latest sign that the once-vanquished childhood disease is making a comeback as vaccination rates decline.

The measles outbreak first reported in Gaines County, Texas, has doubled to 48 cases in the last four days, the Texas Department of State Health Services said in a statement Friday.

Nine of those infected were hospitalized and more cases are likely, health authorities said. U.S. vaccination rates for measles are falling, according to the CDC.