Saliva plays a pivotal role in the health of the mouth and contributes to host defense mechanisms through physio-chemical barriers, non-specific (innate immunity) factors, and specific (adaptive immunity) factors. Of key importance are the specific factors, which include the immunoglobulins that increase in number when there is exposure to cariogenic bacteria.
While one could list a group of diseases that affect dental caries or one could simply categorize the diseases through a single category that involves simply those that either through direct involvement or through treatment affect salivary function and salivary flow. Anything that reduces salivary flow will surely increase dental caries. Caries attack rates in the absence of saliva can be 10-20 times as great as a normal individual.
An example of a condition like that could be, for example, a cancer of say the parotid gland or in the head and neck area where radiation treatments are used to treat the disease. The radiation treatments will destroy the salivary glands and their ability to function, so you inevitably have reduced salivary flow and enormous caries attack rates, in one month equivalent to what would normally be seen in perhaps two years in an individual.
Other diseases could be those that affect renal function and diabetes, or anything that causes the perception of dry mouth or the medications that are used to treat any disease. They may also reduce salivary flow and it’s recognized that the majority of medications that are prescribed for medical reasons have the ability to reduce, or the side effect of, reducing salivary flow.
So patients on medications are almost certainly at risk for increased caries.