Measles and scarlet fever were differentiated from one another in the 17th century. Rubella was accepted as the third distinct pediatric exanthem (rash) in 1881. What were thought to be two distinct forms of rubella were then described and it was proposed that one of these forms of rubella was a separate entity that was called fourth disease. For many years, fourth disease was considered a non-entity, perhaps a mild form of scarlet fever, but certainly not a distinct disease. Now it is clear that fourth disease is caused by exotoxin-producing Staph aureus.