Who counts as my co-workers? I work in a company with several hundred employees in the building, if they were all to lick me it would take rather a long time.

Your immediate co-workers. So the people in your same office, or if that's only you, then your immediate staff. The people you work with often.

my co-workers include my wife and my neurotic friend who would never do such a thing, so I guess I won't be drinking from a toilet :P