As measles cases surge in the U.S. and around the globe, the CDC is issuing new guidance to travelers, encouraging them to get vaccinated for the disease and postpone travel plans if they have not done so yet.

After investigating 50 of those travelers, the CDC was able to determine that measles spread during air travel in at least one of those situations, the spokesperson said. That is usually based on connecting confirmed cases of measles among airline passengers sitting near each other.