Dry Mouth

It is not common knowledge that dryness of the mouth can cause cavities. Saliva does several things in the mouth.  It helps to bathe the teeth so that the plaque cannot stick as easily and it also assists in keeping the ph at a more basic level.  It also helps the teeth absorb fluoride.  When we eat sugars it lowers the ph in the mouth to an acidic level almost as strong as stomach acid.  It takes the saliva at least 20 minutes to bring the ph back up to a more basic level.  If you don't produce adequate amounts of saliva, it makes it easier for the plaque to eat away at the enamel and exposed root surfaces.  
Dentin is the substance that makes up the roots of the teeth and it is softer than enamel.  There are many reasons for gum recession (the gum moving down and exposing the dentin) and that leaves the roots vulnerable to cavities known as root decay.
There are lots of reasons for dry mouth.  Some people just don't produce a lot of saliva, in rare cases, none.  Other people take medications for depression, high blood pressure, allergies, pain, and more.  These medications can cause dry mouth and we are seeing more and more decay related to medication use.  
With the upswing of drug and prescription medication abuse, root decay is being seen more often in younger people.
Sounds hopeless, right?  It's not.  There are products on the market that help to relieve the discomfort of dry mouth.  Mouth rinses, sprays, gels, toothpaste, and gums.  Most of these can be purchased at the local drug store and many can be bought on line.  In extreme cases your MD can prescribe medication.
It's not just uncomfortable, dry mouth can destroy the hard structures of the teeth.  Good home care and dry mouth products can literally save your teeth.
For more in depth information, check out the Mayo Clinic's website.http://www.mayoclinic.com/health/dry-mouth/ha00034