Romans 5:12 — Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned.
One act of disobedience by one man was sufficient to allow sin to enter the world.
Wherefore — refers back to justification in Christ — As justification, so also Adam's sin.
Sin and death entered with Adam, and all sin in Adam, but all also sin on their own.
In Adam — entrance of sin
In Moses (Law) — knowledge of sin (But before the law, before the knowledge of sin, sin still brought death.)
In Christ — forgiveness of sin
1 Corinthians 15:45 — And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit.