Measles is a respiratory disease caused by a virus. It can have life-threatening complications. The CDC calls it the "most deadly of all childhood rash/fever illnesses."
It spreads easily, but the measles-mumps-rubella (MMR) vaccine can prevent it. The CDC recommends that all children, and some adults, get the MMR vaccine.
The U.S. declared measles eliminated from the nation in 2000, but there have been outbreaks since then, and it's still common in other countries.