If they are very infectious, for instance chicken pox just popped up, then they shouldn't be around people.
If they are just off sick for the usual coughs or colds then i wouldn't be surprised to see them at the supermarket. DH is off sick and has walked to the doctors and popped in Sainsbury on the way back for medicine and soup. If it had been ds off with one of us then we would have done the same albeit him on the buggy.