Random Riddle: The Ladder - Common Sense Evaluation

I’m called by three letters
Though I have a long name.
I’m in all of you,
But I’m never the same.

I’m all coiled up
So that I am quite small,
But if you stretch me out
I’ll be really tall.

I could be the root
Of certain disease;
If man can unlock me
He’ll solve many mysteries.

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