An old family friend was a brilliant engineer. He then had a stroke and recovered (early 50s), but within a few months started acting strange. He started making really inappropriate statements, and generally acting weird, started buying commemorative coins and not paying his bills. Then he started talking gibberish and couldn't understand people. His motor functions eventually deteriorated and he passed away a few weeks ago. Absolutely horrifying. By the time we all figured it out, it was a couple of years in.