IMO,
"SIN" refers to two closely related things.
ORIGINAL SIN. Here, "sin" is a brokenness, a fallenness, a spiritual sickness. It's now a part of our fallen human nature. Sometimes this is what is meant in the Bible by "sin" (in the singular) or "sinful."
ACTUAL SIN. That's what it's called in theology, but it doesn't mean only this is real, it means this is how sin is actualized. These are the symptoms of original sin. Often this is what is meant in the Bible by "sins" or "transgressions" (typically in the plural). Here it is "disobedience", a violation of the will of the Lord, in thought or word or deed, by what is done and left undone, intentional or unintentional.
Let's say I have a cold (I don't but play along). That's the disease. I HAVE it. Now, if I cough or sneeze - those are symptoms of the cold - they cold brings it about, manifests itself by such. The cold causes the cough, the cough doesn't cause the cold. Now, it's possible that at this moment I have no symptoms but I may well still have the cold.
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