What exactly is original sin, and are newborns born with sin?

Adam and Eve's eyes were opened immediately after Adam took the bite. This is where sin first entered the human race. The concept of original sin implies that all humans are contaminated by sin as a result of our first parents' disobedience. We are all morally and ethically wicked to the point where no one is by nature good including babies, except Jesus Christ. God has granted infants privileges to enter the kingdom of heaven until they reach the age of reason, given that they are incapable of repentance until then. Some theologians call this special grace. 

Bible References: 

Gen. 3:7; Rom. 5:12; Psa. 51:5; Lk. 18:19; Matt. 19:14; Isa. 7:15-16