Depression may spark Alzheimer's disease, researchers believe. People who have suffered depression are twice as likely to develop Alzheimer's, and the risk is higher if you have recently suffered depression.
If depression has occurred within the past 12 months, you are four times as likely to develop Alzheimer's as someone who has had maintained a positive mental attitude; beyond a year, the risk falls to just one times, and the association is merely 'modest' if depression happened 25 years ago.
(Source: Archives of Neurology, 2003; 60: 753-9).