I'm really scared my grandad might have Alzheimer's?

he's having a test tomorrow i think, a glucose test? or something. And some lady is coming round in a fortnight from our local mental hospital. At first im afraid to say i didn't take it serious i didn't really know what it was, my nan was convinced he had dementia cause he forgot things at the supermarket so at first i found it strange... but i read about Alzheimer's- and I don't want to witness my grandad- who is a lot healthier than my nan who could more or less go any day now- slowly die and have to be cared for. I love my grandad he's probably more active than me and i'm 15! lol he doesn't sit in a chair he makes me laugh and messes about I just don't want to watch him deteriorate into nothing! I'm scared... i don't want to loose both of them even though it will happen some day im okay about my nan cause she most likely will p peacefully but not m grandad