Classified info:
Seth Martin Brundle was born on Earth. He was a brilliant man. He was especially interested in matter transportation. He'd always offer to help fix transporters on starships that docked near his town. However, one day, he was testing a transporter for any further bugs, and a terran housefly was transported with Brundle on his test. The transporter was unfortunately still not working correctly, and Brundle's genes were fused with the fly's. At first, he felt much better, but then he started to deteriorate. Doctors were able to stop the degeneration, but he was already halfway to becoming a monster.
Interestingly, Seth's direct decendant, for whom he was named, was a pioneer in matter transportation technology, and had the same accident. The original Seth had transformed fully shortly before his death, and his son Martin supposedly washed out the fly genes from his bloodline. Today's Seth, being named for both these ancestors of his (Seth Martin), feels that this was destiny, although sick and twisted, claiming that there was still fly genes in his bloodline to begin with that the original Martin missed (which would account for the fact that today's doctors could only stop the degeneration, and not reverse it).
Seth is just as eccentric as his namesake was reported to be, prompting some experts to suggest re-incarnation. These experts have been locked away in mental institutions.
The original Seth coined the name "Brundlefly" for what he became. The modern Seth likes the classification, but still prefers to put "Human" on his resum�s.