When rugs stay wet too long, they become mildewed and, eventually, suffer dry rot. The classic example is dry rot caused by a potted plant placed on a rug. The typical result is a horribly rotted circular area in a carpet that is otherwise in good condition. Don’t even think about putting a potted plant on a rug. No matter how clever you are, no matter that you use a glazed pot and a glazed saucer and you put a vapor barrier between the saucer and the rug, the rug will get wet and will stay wet unbeknownst to you and will become a rotten mess in an area about one foot in diameter.
Another typical situation comes up when rugs are stored poorly, in a garage for instance, and they become wet without anyone realizing what has happened. Even though dry rot is not inevitable in such cases, a mildew smell is, and sometimes the smell of mildew simply cannot be removed. I have seen several occasions when moisture under a house has caused rugs on the floor above to mildew.
Please do not worry needlessly, though. A little water on a rug, or even a lot of water, will not cause it to mildew unless the rug stays wet too long. For instance, when a rug is stepped onto after a shower or bath rarely are hurt by water because they have time to dry out between bath times. And don’t panic if you spill a glass of water on a rug. Just dry it as well as you can with towels.
Unfortunately, besides causing mildew and dry rot, water sometimes causes dyes in rugs to bleed or run. All you can do in this situation is to get the rug dry as soon as possible and to send it to a expert rug washer. If a rug is just a little wet, as from a spilled glass of water, do what I suggested above. Merely soak up as much water as possible with a towel or paper towel and everything will probably be just fine. If you are worried about the floor underneath, elevate the wet spot until it dries.
A rug that is thoroughly wet is another matter. The goal is to dry it before it mildews in about four or five days. If you have a Shopvac or other vacuum that will take in water, vacuum out as much water as you can. If you have sunlight and a place to lay the rug, open it and let it finish drying outdoors. Or, if you know that the rug is dirty as well as wet, dry it enough so that you can get it to an Oriental rug cleaning specialist.