Infections can be picked up from all sorts of places, being in shops and breathing others peoples' germs, a scratch that doesn't get washed quickly enough, grubby worktops, face flannels which arn't boiled regularly etc. etc. Just tell you gran to keep an eye out in hospital and make sure the nurses and doctors all wash their hands before doing ANYTHING to her, to avoid MRSA infection and if you visit, make sure you wash your hand too before entering her ward. The elderly are particularly vulnerable to these kinds of infections so everybody needs to take special care around them.