Measles can spread easily from person to person, but it is a fully preventable disease. The World Health Organization notes that a safe, effective vaccine has protected billions of children against measles over the past 50 years. However, all that progress is now at risk. Katherine O’Brien is the director of Immunization, Vaccines and Biologicals at the WHO. She says that progress is at risk because of a failure to vaccinate children in all parts of the world. Measles as a virus is one of the most contagious infections that there is. For every case of measles that occurs in a setting where people are not immune, nine to 10 additional cases will occur simply because of exposure to that case." Measles is a virus. It lives in the nose and throat mucus of an infected person. “It can spread to others through coughing and sneezing,” reports the U.S. Centers for Disease Control and Prevention.