Measles is an infection caused by a virus and is most common in children between the ages of one and four.
It's infectious and spreads through coughing and sneezing. The symptoms are a runny nose, sore eyes, cough and fever.
A rash then appears and it can cause diarrhoea, being sick and tummy pain. Usually it clears up after a couple of weeks.
But in some cases, it can be dangerous for children. It can cause a very bad cough, breathing problems, ear infections, pneumonia and eye infections.
On rare occasions - one in 100,000 cases - it can infect the brain and cause brain damage.