Once the water is gone, the dry wall will have to be removed to get to the mold behind the walls. Mold is a living thing that must be killed using chemicals to prevent it from multiplying and becoming even worse. Before repairs occur, usually big fans are used to dry out the home removing the moisture that caused the mold in the first place.
Mold can occur from any plumbing leak not just flooding by a natural disaster. Once there is sitting water anywhere in the home, it must be checked for mold. Mold is a major health hazard that can't be ignored. If it results from flooding, depending on circumstances, it may be covered by home insurance; but check your policy for the fine print. Either way, mold must be remediated to continue living in a home. There is no other choice.