New research presented in S@#?%&* Translational Medicine suggests that Alzheimer's Disease can develop in response to a microbial infection. Scientists at Harvard Medical School showed that the amyloid plaques associated with Alzheimer's were able to protect against meningitis in mice.
Aβ amyloid fibrils were produced in the brain and bound to invading pathogens, eventually encapsulating them in a "prison".
This has never been show in humans and whether or not infection could really be a clinical cause of Alzheimer's Disease remains to be seen. At the very least though, inflammation pathways could now be investigated as new targets for the treatment/prevention of Alzheimer's.
Image attribution - Neil Conway used from Flickr under Creative Commons.