Professor Dennis Selkoe points out that although Alzheimer's disease is primarily a genetic disorder, environmental factors do contribute.

We’ve known for maybe 30 years, 40 years, even 50 years that there are genetic factors that lead to Alzheimer’s disease, but the question comes up ‘what about the Alzheimer’s [disease] patients who don’t have a clear cut family history?’ There are many people who get Alzheimer’s [disease] in their 70’s, 80’s and beyond, and there is no knowledge that anyone else had it. We think that either that means it’s a complex genetic trait, and no one gene is simply responsible and so it isn’t a simple linear genetic inheritance, and/or there is an environmental factor that led that person to have Alzheimer's [disease] without a family history. I emphasize this issue of family history because actually most of the evidence suggests that genetics is the number one factor that leads to Alzheimer’s [disease], and environmental factors seem to be less crucial. But it could well be, and I think it is, that certain genetic pre-dispositions don’t automatically cause Alzheimer’s disease 100% of the time, but interact with an environmental factor.

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