Study suggests that memory loss in Alzheimer's patients may be reversed 12 years ago
There is some evidence of a correlation between oral hygiene and Alzheimer's:
http://www.medicalnewstoday.com/articles/264164.php
Basically, people with poor oral hygiene are more likely to develop Alzheimer's than people with good teeth.
You might have heard about a similar study that found a link between oral hygiene and heart disease. The theory, at least in that case, is that bad oral hygiene causes chronic long-term inflammation throughout the body, not just in your gums. This might increase risk of heart disease. Without looking at the data, I would guess that the same theory applies to Alzheimer's and maybe other diseases. The usual caveat about correlation != causation applies of course.