Mildew is a destructive fungus that feeds on organic materials.   Mildew is considered a type of mold, and while it is often thought of as a single organism, there are many types of mildew that grow on a variety of surfaces, from wood to paper and wool and cotton which most area rugs are made of.  While mildew is associated with a musty odor and splotchy stains, it is actually odorless and invisible, the odor and stains are a result of its digestive process as it eats through its food source.  Mildew exists in a dormant state throughout the environment and only under the right conditions, does it begin to spread.

Mildew grows when there is a food source and moisture, and if there is a lot of moisture and warm temperatures it can grow quickly.  When mildew grows on a rug it can weaken and damage the fibers.  If mildew goes unchecked it can leave permanent stains and even rot away entire sections of an area rug.  Once a fiber has been exposed to mildew any damage will be permanent.  It may not be as extreme as a gaping hole or large, splotchy stains, but even a small amount of mildew can weaken any part of your rug it comes in contact with.

The only way to prevent mildew growth is to keep area rugs dry.  Under the right conditions, mildew can begin growing in a matter of hours, so if a rug has been in a flood it should be dried, and ideally professionally cleaned, as soon as possible to prevent mildew growth.  High humidity levels can also cause mildew to grow.  If you have rugs that are being used, or stored in areas with high humidity they should be moved, or checked regularly for mildew.  Another option is to use a dehumidifier in the storage area to help keep moisture levels in check and protect your rugs and any other possessions in storage. If there is mildew present in your rug, there are a number of fungicides that a professional cleaner can use to treat your rug.  

Keep in mind, any damage caused by mildew is permanent and your cleaner cannot reverse the effects of mildew.  Sometimes a rug will be weakened enough that cleaning will damage it further, and additional repairs are necessary.  Your cleaner can consult with you about what condition your rug is in and what would be appropriate.  After your rug has been cleaned and treated, any prevention of further mildew growth depends on the environment your rug goes into.  If the conditions remain conducive to mildew growth, your rug may continue to develop mildew and be subject to the rot that comes with it.