A professor of neurology recently wrote.... "we should probably assume that a significant proportion of those over the age of 65 have cognitive impairment, one form of Dementia or another"

Dementia is not really a disease but rather the normal age-related changes to brain structure. Our bodies are slowly breaking down as we move towards death. I assume we call it a disease since it doesn't affect everyone, but in actual fact it does, just not to the same extent.