This is a stigma against an individial who hangs out with an unheigenic person. For instance your friend points out to you , ew OMG that guy you are with has herpes on his face! Then they think you will become dirty or infected if you keep hanging around them. Herpes is transmittable so you have to cut the ties! Youll get a terrible reputation hanging around with a herpes kid.
OMG, that Vitale kid has herpes. Don't associate with him or you will have herpes by association!
This is when herpes can be contracted simply by having skanky friends who are contaminated.
Eww. (Insert name here) is friends with (insert name here). She totally has herpes by association.