MANY health authorities are now taking the position that it is much better for an individual to have mumps while a child rather than postpone the attack until adult life when complications are more prone to occur. It should be remembered, however, that this position is not taken by any health authorities with reference to any other of the contagious diseases. Mumps, as a rule, is not serious in children. In adult life, however, a painful complication, inflammation of the reproductive glands, does sometimes occur. It is thought that fatigue, chilling, over-exertion or a slight injury to the reproductive glands may contribute to secondary infection.
People who have had MUMPS are considered immune
FLORIDA STATE BOARD OF HEALTH
State Archives of Florida: Series S908, Box 01, Folder 11
Pamphlet about Mumps, its causes, how it's contracted, how it can be avoided, and common misconceptions about creating immunity.