It's probably not a good idea for op to try to diy this from a few sentences off the internet then...

Any mold inside is a biohazard to some degree

Possibly, though I have seen it not be the case before. Had a leaky bathtub drain that rotted a bit of the ceiling, pulled out the drywall, the floor sheeting around the drain was fine and so were joists, it had been dripping down the u bend just onto the drywall below. Sometimes you get lucky and things aren't as bad as they seem. Not always

1. Cut out as much of the rotted stuff as you can that isn't structural

2. Identify the source of the leaking and stop any leaks, or if you are unable to handle that yourself, turn off the water to the toilet and drain it until a more qualified person is able to do so. If that is the only toilet in the house you should not be using it anyways and this is definitely an emergency situation that you should reach out to any and everyone possible to get fixed asap.

3. Once the leak is stopped you can asses the damage to the structural portion and remediate. This is where further advice becomes useless without having actually seen the situation