But while researchers are not certain about the cause of this form of dementia that typically occurs in old age, they have noticed some associations between certain lifestyle factors and biomarkers and the risk of Alzheimer's disease.
Below are nine factors that, sometimes for unknown reasons, seem to foretell a higher risk.
The risk of dementia increases with age, and so Alzheimer's is more likely to affect someone as they get older. The risk of Alzheimer's increases after age 75 and increases even further after age 80.
The 3MS exam is a series of questions and answers that helps determine cognitive function and is one of the diagnostic tests a doctor will use for Alzheimer's.
"It's more of an interview, really," said Dr. Sam Gandy, chair of the National Medical and Scientific Advisory Council of the Alzheimer's Association, noting that the test does not look at executive function or spatial ability.
A lower score on this test indicates a higher risk for Alzheimer's.
The DSST requires more abstract thought than the 3MS and is generally considered a better evaluation test in people with higher levels of cognition. It involves showing patients various digits and symbols and describing them to the examiner in their own words. Lower scores on this metric indicate a higher Alzheimer's risk.
Maintaining a healthy body weight is important in preventing Alzheimer's disease, and it seems that works both ways. Having a BMI below 18.5, the lower end of the "healthy" range, is associated with an increased risk for developing the illness.
While there is no definite genetic test for Alzheimer's, a specific variant of the APOE gene, e4, has been associated with an increased risk. It remains unclear how they are connected, but the present theory is that it inclines people to accumulate amyloids on the brain.
"There are more people with Alzheimer's because they have [that gene] than with any known genetic risk factor," said Gandy.
He said that half of all people with the Alzheimer's have the gene. While people may have zero, one or two copies of the gene, with each copy tripling the risk, Gandy noted that some people with two copies in their 80s and 90s still do not have Alzheimer's, "probably because they have some strong protective genes."
The presence of white matter on an MRI is associated with increased Alzheimer's risk, although scientists are not entirely sure how the two connect. One theory is that the blood-brain barrier is not entirely intact, owing to the presence of amyloids.
Enlarged heart ventricles, thicker carotid artery walls and a past coronary bypass surgery can all signal a higher Alzheimer's risk.
"This is an area of a lot of recent interest," said Gandy. "It's clearly a type risk between ... risk of atherosclerosis and risk of Alzheimer's disease."