Dad was recently diagnosed with dementia. Last summer he was able to pay his own bills, comprehended probably 99% of every conversation, good appetite, worked in his yard. Now just over four months later, he barely eats, wants to sleep all the time, days go by with him talking to himself as though he were at work, he sees people who aren't there that no one every knew, and he sees family members who have passed, but he still knows our immediate family when he sees us, but gets our names mixed up, he's always done that though. Is it common for dementia to get this bad this fast? I've heard it takes years and years for it to progress.