A common cause of mold and wood rot issues in bathrooms is a leaking tub. Depending on what type of tub you have in your home, the leaks could be depositing water either on your floor or directly into the walls and floor. If the second situation is true in your case, you might not even get any warning before the damage is done. There are a few areas that are more likely to be the problem than others. For example, the caulk and grout around your tub could crack and start allowing water to pass through. You should regularly check the state it is in and have it repaired as soon as possible.
Clogged pipes can also cause an issue where water backs up and finds its way out of the drain system. If you notice that the water in your tub drains slowly, it is probably time to have it inspected and snaked.