Rubella is also called German measles. It's caused by a virus and spreads when an infected person coughs or sneezes.
Usually rubella causes a rash, slight fever, aching joints, headaches, runny nose and red eyes. The rash starts on the face and spreads from head to toe.
Sometimes the lymph nodes behind the ears and on the neck can swell painfully.
But lots of people don't get these symptoms so it can be hard to tell if someone has it.
Rubella can be dangerous for pregnant women in the first three months of pregnancy.