There's news today that beta amyloid (A-beta) may not be a cause of Alzheimer's Disease, but "may be part of the brain’s normal defenses against invading bacteria and other microbes." If the researchers are "correct about A-beta being part of the innate immune system, that would raise questions about the search for treatments to eliminate the protein from the brain. ... But does that mean Alzheimer’s disease is caused by an overly exuberant brain response to an infection? That’s one possible reason, along with responses to injuries and inflammation and the effects of genes that cause A-beta levels to be higher than normal..."

See:
http://www.nytimes.com/2010/03/09/health/09alzh.html