CDC tells Americans to cancel their flights after finding world's most infectious disease is spreading on planes | 31 May 2025 | The CDC is urging Americans to cancel their upcoming flights if they are not vaccinated against measles. The agency has upgraded its health travel warning after discovering the virus, one of the most infectious in the world, is spreading on U.S. airplanes. A previous recommendation that said Americans should "consider making alternative travel plans" if they could not get vaccinated before departing now says travelers should "consider postponing their trip." After investigating dozens of patients who were contagious while flying, the CDC confirmed at least one case where the virus was spread during air travel.