I've actually had an experience with what I believe was my body creating a bug. I'm convinced my body created campylobacter to clean me out when I was at my sickest. I had a stool sample checked out in a lab and that's what they found. It's hard for me to believe I caught campylobacter as I was fasting on bone broth/juiced greens at the time. Also my body felt so much better when it was gone.
Of course I could never prove that I didn't eat something to poison myself, just sharing what I'm convinced of through my own experience.