An unexpected random event
He was Luke the Fluke, but you didn't call him that.
Occasionally the fluke migrates into the blood vessels and may reach the lungs, kidneys, urethra and bladder.
The body contains in miniature all the organs of the adult fluke, including the gonads and in addition "eye-spots," a stylet, rod-cells and cystogenous cells.
When that occurs, the cyst is dissolved and the minute fluke works its way down the alimentary canal into some part of which it inserts its suckers and commences to feed on the blood of its host.
As for Luke the Fluke, he sounds like John Luke Grasso to a tee and a prime suspect for killing my friend's sister.