Charlie,
EDIT: Also, old people rarely run a fever when they have an infection but rather, it effects their temperament. Make sure the doc does a blood workup. Bladder infections are most common (usually women but also men if they don't drink a lot of fluids) and every time my Mother-in-law got weird, they would find a bladder infection and she's be fine after a couple days of antibiotics.
EDIT Again: Older folks dehydrate easily because they stop drinking as much fluids because it's hard for them to visit the bathroom. Low order dehydration causes temperament changes...
Ray