I once had a teacher whom a classmate of mine described as a Dementor (you know those characters from Harry Potter who suck out your soul and steal your joy?). This person was actually very nice, but scared the piss out of you with just one disapproving look.
The word dementor comes from the same Latin root as the word for dementia (de- meaning "away" or "apart from", and mens/mentis meaning "mind"). So literally, the word dementia means "taking your mind away". Dementia is though of as a decline in your cognitive abilities beyond what we normally allow for age. It is not just forgetting your keys or where you parked your car. It simplistically means forgetting people, places and things that you have known for years.
Far and away the most common single cause of dementia is Alzheimer's Disease. It can also occur after a stroke or head injury (see what happens to boxers after many years in the ring). Alzheimer's Disease (AD) is caused by the accumulation of a protein called amyloid in the brain. Over time the amyloid destroys the brain tissue and makes things like logic and memory problematic. No one knows the entire cause, but there are some risk factors:
-family history (particularly for those with early onset AD occurring between 30-60 years of age)
-carriers of a particular protein called ApoE (we all have it, but about 10-15% of the population has the form that increases AD risk
There are some studies that suggest that high cholesterol, smoking, high blood pressure, poor diet can increase the risk as well. There may be environmental factors, like supportive social structure, low stress, mental activity and physical activity that decrease your risk of AD. Some studies found that those who are multilingual and those who do crossword puzzles regularly have a lower risk of Alzheimer's. They go by the theory that the brain is like a muscle. Use it or lose it. Having a family history doesn't mean that it is inevitable, but there are lifestyle changes that can be made to lower your risk. It would be something to possibly talk to a genetic counselor about, but it obviously has great financial and social repercussions.
Great, now I have to go for a run while listening to Italian language lessons on my iPod and find a copy of the New York Times crossword. Oh never mind, I forgot that I'm allergic to exercise. And I have no idea what jojo's disease is.