The Centers for Disease Control and Prevention (CDC) is issuing this Health Alert Network (HAN) Health Advisory to inform clinicians and public health officials of an increase in global and U.S. measles cases and to provide guidance on measles prevention for all international travelers aged ≥6 months and all children aged ≥12 months who do not plan to travel internationally. Measles (rubeola) is highly contagious; one person infected with measles can infect 9 out of 10 unvaccinated individuals with whom they come in close contact.