Measles, or rubeola (part of the MMR vaccine) is caused by a virus. It is spread very easily through coughs and sneezes, McCarthy says. The Centers for Disease Control calls it "the most deadly of the childhood rash and fever illnesses." McCarthy says it causes a fever and a rash, and can lead to brain infection, pneumonia, or even death. "There is no treatment. Recently we have been seeing cases of it in people who haven't been immunized or have had only one dose of the vaccine."