Due to a poorly fastened toilet drain flange - screws were not rust proof- the seal under the toilet allowed water to leak out and begin soaking into the sub floor, exacerbated by the use of an inferior product: particle board. When particle board comes into contact with water it wicks it up like a sponge and then expands and ultimately disintegrates. Because this material was under sheet vinyl, the water could not evaporate. This lead to mold, which is never good, especially when the client has breathing problems.
The old floor was removed including the original vinyl, new plywood was installed and the perimeter and seams caulked to prevent water entering underneath in case of a flood. Finally, before the trim boards were reinstalled more caulking was applied for even more waterproofing.
Correct materials, and a little bit of attention to details can lead to a much better job and doesn't add much to this type of installation.
