[jeff5may]

Yep, you've got a decent mystery to solve here, sir. This is not one of those situations where you can throw on a quick patch or band-aid and cover the affected area and simply let it heal on its own. If not done perfectly this problem will come back to haunt you with regularity and increasing expense.

Since the area is under a bathroom, I would rip out all the drywall directly below any bathroom fixtures (sinks, tubs, showers, toilets,etc) and inspect the top sides of the removed sheets for moisture or staining from moisture. Just because your panels rotted apart at the seam doesn't mean that's where the water came from. Drywall is relatively inexpensive as far as building materials go, so don't be afraid of taking too much out.

Once you have removed the wet drywall and identified and repaired the source of the moisture, make sure the enclosed cavities in between floors has had ample time to dry completely and that there is no mold anywhere in there. If there is still evidence of mold growth, those areas should be treated to remove any possibility of regrowth. It only takes a few spores to repopulate the entire area, so be thorough in your eradication.

If you get in there and can't find any obvious leaks, you may have an infiltration or migration problem. Water can follow piping for a very long distance if the pipes have much slope in them. However, it always leaves a trail, so finding the source is not super difficult. OTOH, the actual repair may be incredibly difficult and frustrating for a novice to perform correctly. Don't be ashamed to call in a pro if the repair looks daunting.