When two people who don't have an STD have a threesome with a person who does have an STD; the infected participant is the patty and the two non-infected participants are the buns in said Saucy McMuffin.
Paul: "You do know that she has chlamydia right?"
Will: "Well shit, looks like Steve and I were the buns in a Saucy McMuffin."
Paul: "Yeah, you two were super drunk last night. You should probably get tested. There's a clinic over on Hillside."
Will: "You're right, it's probably for the best. I'll text Steve too."