How do you clean mold and mildew caused by a flood in the basement?

Your best chance of getting rid of the mold and mildew is to use bleach to clean it. Mix the bleach with water according to the directions on the bottle, and you can also add a few drops of dish detergent if you want. Although you didn't state what type of surfaces you need to clean (concrete floor, walls, etc.), bleach can safely be used on many of these surfaces. Apply, let set a few minutes and rinse well. Depending on the type of surface, and the severity of the mold and mildew, you may have to scrub a little to get rid of all of it. If you are unsure of a particular surface, test a small inconspicuous area first, just to be safe. If there are areas or surfaces that you know you cannot use bleach on, try mixing table salt with enough water or lemon juice to form a paste, then scrub the area or surface with it, and rinse.