Surprisingly common. My stepdad didn't have a stroke but his immune system did start attacking his brain as a sideeffect of leukemia. He used to be very progressive but now he is noticeably brain damaged and a big fan of fox news. Pretty telling that the only thing I know of that gets people to watch fox news is actual brain damage.
Oooo I might place a bet on that now. He'll carve out an identity as rejecting progressive ideas, as he is now, and the DNC will run him as the "hope and change" candidate