Two proven risk factors for Alzheimer's disease are age and family history. The disease usually strikes after age 65, and risk increases with advancing age. Having a family member with Alzheimer's increases one's risk, particularly if the relative has the early-onset form of the disease (beginning before age 65). However, half of people with the far more common late-onset form have no family history.
Many non-genetic - so-called "environmental" -- factors may also increase one's risk, and a great deal of research is aimed at understanding these factors. Examples include a former serious head injury, lower levels of formal education, and things like diet, exercise and a healthy lifestyle - but scientists aren't sure why or how these things interact to produce the disease in some people but not others. There is recent evidence that one's environment and experiences early in life may also play a role in the eventual development of Alzheimer's. Research is ongoing to try to solve these mysteries.
Learn more about Alzheimer's by visiting National Institutes of Health's Alzheimer's Information Page (opens in new window).