Pretty simple theory here that mainly tries to reconcile the "Sarah was the frogbug girl" theory with the "Judy was the experiment" theory.

I'm thinking that, unlike the BOB and Laura orbs, the frogbug was something very simple and either lived in Sarah granting her the ability to have visions and channel Earle and the like, or simply made changes to her before moving on or dying or whatever. This would basically make Sarah a suitable host for potential future use. I definitely agree with those that don't buy into Sarah being possessed by ultimate evil throughout the entire original series. It seems far more likely that Sarah wasn't hijacked until the experiment broke out of the box and that the bug was a means to make this possible.

Anyway, just a thought.