Apparently, people who care for a spouse with Alzheimer's are themselves at a higher risk of developing dementia.  Some undiscovered disease?  Environmental toxins?

Actually, the scientists think it may be because, well, caring for a spouse with Alzheimer's is really stressful.  And stress takes a toll on the brain.  The main evidence for this proposition?  Male spouses have a much higher risk than female spouses:  12 times the normal risk, compared to 4 times for women.  Men in general do worse when they lose a spouse, because (at least in the generation now at risk) they often aren't used to taking care of themselves, and they tend to have poorer social networks outside of work.  So having to become an intensive caretaker is probably more stressful than it is for women, who are used to it.