A BBC reporter, Melissa Hogenboom, talks about her father in this BBC article "The part of my dad that dementia can't take":
Is this 'why' a fused relative word?My dad does not fall into any of the risk categories for dementia. There is no family history of it. He’s always been slim, healthy and active. This is part of why the disease is so devastating – it can affect anyone and we still do not understand why.