Ronnie: Be afraid. Be very afraid.
Seth Brundle: My teeth have begun to fall out. The medicine cabinet is now the Brundle Museum of Natural History. You wanna see what else is in it?
Seth Brundle: I think you're making a mistake. I think you really want to talk to me.
Ronnie: Sorry, I have three other interviews to do before this party's over.
Seth Brundle: Yeah, but they're not working on something that'll change the world as we know it.
Ronnie: They say they are.
Seth Brundle: Yeah, but they're lying. I'm not.
Seth Brundle: I was not pure. The teleporter insists on inner pure. I was not pure.
Ronnie: I don't know what you mean.
Seth Brundle: A fly... got into the... transmitter pod with me that first time, when I was alone. The computer... got confused - there weren't supposed to be two separate genetic patterns - and it decided to... uhh... splice us together. It mated us, me and the fly. We hadn't even been properly introduced.
Tawny: Are you a body builder, or something?
Seth Brundle: Yeah, I build bodies. I take them apart, and put them back together again.
Ronnie: Don't you get it? I am finally onto something that's big. Huge.
Stathis Borans: What? His cock?
Ronnie: Crude Stathis. Even for you.
Seth Brundle: There was an old lady who swallowed a fly, perhaps she'll die.
Ronnie: If you SAW him, Stathis, if you saw how scared and angry and desperate he is.
Stathis Borans: I'm sure Typhoid Mary was a very nice person too when you saw her socially.
Suggested correction: Being as how the bacteria and mites and such were IN or ON Seth, the machine was able to organize those symbiotic relationships accordingly as teleporting one would teleport all. The fly was separate, not touching. The machine was not programmed to anticipate two separated entities and so combined them into one on the other side.
Phixius ★
We leave behind a vapour trail of bacteria and viruses (among other things) as we walk, in our breath and emanations from pores in our skin, and Brundle isn't trapping any in his clothes as he isn't wearing any. Brundle has an invisible cloud of DNA floating around him in that teleportation chamber and as far as the machine is concerned their DNA has exactly the same status as that of the fly.