What is Measles? Measles is a highly contagious disease caused by a virus. It spreads quickly when an infected person breathes, coughs, or sneezes. It can cause severe disease, complications, and even death. It infects about 90% of unvaccinated people who come into close contact with an infected person.   Measles can affect anyone, but it is most common in children. Vaccination is the best way to prevent getting sick with measles or spreading it to others. The vaccine is safe