People who are connected to each other via the transmission of an STD.
BOB: Jane is such a fox!
JOE: Yeah, the sad part is, we are part of the same STD chain, but, I have never actually fucked her.
JOE: Jane gave herpes to Kate, that ugly bi chick I fucked last summer when I was waisted, and Kate gave me herpes. So Jane and I are in the same STD chain.
BOB: Oh, I get it! Cool.