I'd rather know, so I can take the proper precautions. I would really resent getting sick because someone didn't think it was important to mention they were contagious, and because most people are very uneducated about transmission... so if a friend with less than stellar hygiene invites me over for supper and she was sick previously, I'd like to know and postpone until later or propose to eat out somewhere instead of freaking out afterwards about an almost 100% chance of infection.