As I sit here, waiting for my next "call to nature", it occurs to me that the only times I've seen illness / viruses / flus and the like in an RPG campaign, it been as part of a plot hook.
I don't ever recall an adventuring party getting sidelined by dysentery, a flu, the norovirus (love ya dude, but you are more than welcome to take your leave now) or any such run of the mill illness.
All of which leaves me to believe that there must be a series of unpublished spells that take care of such issues. Maybe the magic-user has a "Sterile Cleaning" spell for his workshop or clothes. Perhaps clerics have the "Cure Minor Aliment" spell to get rid of the the of the mill flu or viral infection, and a "Cure Major Aliment" spell for pneumonia and the like. Because really, you need something a bit more flexible than Purify Food and Water.
If there were magical cures and / or preventions, it would all make sense. Maybe I need to put my party up against a group of norovirus infected adversaries and watch the fun that follows...
Things that cross your mind while on a "liquid diet". I can't see myself fighting orcs or slaying dragons in this state ;)