Measles is a highly contagious respiratory disease (in the lungs and breathing tubes) caused by a virus that is spread by direct contact with nasal or throat secretions of infected people (when a person infected with the measles virus breathes, coughs, or sneezes). Measles is one of the most contagious viruses on earth; one measles infected person can give the virus to 18 others. In fact, 90% of unvaccinated people exposed to the virus become infected. You can catch measles just by being in a room where a person with measles has been, up to 2 hours after that person is gone. And you can catch measles from an infected person even before they have a measles rash.