Hi Berberry,
Being raised in a Catholic home and being Catholic I feel I can shed insight. Irish Rock hound's comment is right. OS is indeed "inherited" from the Garden of Eden story in the Old testament. Catholics babtize infants for this reason, the infant is born into sin. Babtism removes the stain of original sin and infuses the child with the holy ghost. The ceremony is a sacrement one of seven a Catholic can recieve grace from. The next is the sacrement of first holy communion. This takes place around age 10-13 then person may receive the sacrement of confirmation by a Bishop of the church.
There is also the sacrement of marrige, the sacrement of holy orders, the sacrement of last rites.
Original sin is a necessary in the Christian faith I believe because the whole concept of redemption hinges on it. Jesus is the redeemer 'the lamb of God' If Adam and Eve never disobeyed God then what need of a redeemer would we have?This is just my opinion. Original sin is present on all infants except for Mary who was born imaculant, and Jesus. It is also presumed by the Catholic church that Mary did not suffer death but was rather assumed into heaven directley. Hope this give you another point of view.
**this is all Catholic teachings and has no basis in fact whatsoever**