You may simply have to remove a few tiles - stripping the floor area surface and relay new ones or it may involve construction that is more extensive. Sub-flooring difficulties can rear up in numerous ways. It's that area in the top where the gap will attract moisture and water, which will trickle down from the bathroom and settle there. It might start out a simple musty smell from early mildew, which will not go away no matter how much you clean. It could be as serious as a dip or a bowl feel under foot in the carpet or tile.
At this point you will have to begin the repair as soon as convenient. Depending upon the kind of over flooring you have installed there are various ways that you can proceed with the repair. If you have installed vinyl floors, for example, you will rarely reuse them. They are almost thought of as disposable floors. They will have to be replaced. Should you have a regular tile floor you can with some care remove a few and try the repair from there. You may have installed stone tiling. You will be able to removes some of these in the affected area with them cracking.
On the other hand if you have a wood floor, you're in a borderline region for repairing the sub flooring area. In the case of wood laminates, they can be reused after you have taken them up to repair the area. However, with the price of laminates today the cost, and the extra effort is usually not worth it. You're better off being prepared to replace the planks. Carpet will definitely have to be replaced - all of it!
Remove any flooring in the damaged area. Then inspect the damaged sub-floor. The most typical problem is water that has rotted plywood. Though never easy, there are worse things to have to repair. Cut away the rotted plywood, removing an entire sheet if possible.
If not, simply cut a large enough area around the damage to ensure that all the rot has been cleared out. Mildew is made of living organisms and they can spread. An exterminator may be rehired to eradicate insect induced destruction. If this destruction isn't stopped it'll continue to do more damage.