The unual occurance is when a large number of people in the same workplace gets sick all at once. Common in the UK, where old buildings have crap for air cirulation systems.
Less innocent is when several people in an office have been sleeping with each other and suddenly they all have the same STD.
Joe: Betty and Rick...even Mike has the crud.
Pete: Looks like we have an Offphilis epedemic.
Joe: Turns out Mike, Rick and Betty had a three-way, and now they all got the herp.
Pete: Shit dude, this Offphilis is legend!