Rubella causes a trivial illness in children and a slightly more troublesome one in adults.
It is serious only when it affects a woman in the early months of pregnancy, when there is a chance that the virus will infect the fetus, which can lead to any of a range of severe birth defects, known as rubella syndrome. Symptoms develop after an incubation period of two to three weeks. Once common worldwide, rubella is now much less prevalent in most developed countries as a result of vaccination programs. By 1986 the incidence in the US had been reduced to 551 reported cases of rubella and only 14 cases of rubella syndrome. The infection usually occurs between the ages of 6 and 12 and is almost invariably mild.