If the water damage done to your carpet is by clean water, then it won't cause staining. Stains aren't the only consequence of water damage. If the carpet was soaked and not dried immediately, then mold and mildew might begin growing on it. You'll know if fungus grows on the surface the carpet: you'll be able to see it and feel its slimy texture when you walk on it with bare feet. Mold can also grow on the padding beneath the carpet, hidden where you can't see it.
Mold growing on your carpet's padding presents several dangers. The spores it releases can trigger adverse health effects in some people, resulting in symptoms like wheezing, watering eyes and skin rashes. Mold releases a foul, earthy smell that can permeate your home. Because it is hidden, it can grow unchecked and spread to other areas. Water damage in one room can eventually trigger mold growth in other areas of your home.
When water damage occurs to your carpet, dry it immediately. Use extractors to pull water from the carpet and the pad beneath it as well as fans to dry the surface. This reduces the risk for mold, but does not eliminate it. You can pull up the carpet at the walls and draw it back to visually search for mold. If you find it in a small, isolated patch on the padding alone, remove the moldy area along with a foot-wide barrier of clean padding. Replace it with new padding.
If you don't want to pull up the carpet, professional mold remediators can test your home for elevated mold spore levels, which indicate the presence of mold under your carpet. If mold is growing on a large patch of the padding or if there are multiple areas of mold growth, the padding needs to be replaced. If mold is growing on the upper side of the padding against the carpet, then the carpet must be replaced too. This protects your family and your home from the harmful effects of mold.