Once the critters have left the digestive track and have been killed you are still left with eggs that have no way to leave your body. A combination of low immune level and solvent build up and bingo...
Take the A.Vasorum I am still dealing with the eggs today, the SCIO even refers to the A.Vasorum (eggs) as the cause of inflammation of my spinal column even though the critter itself now clocks in at low 37 (>80 is of interest).
See my prior post (bio-warfare) from a couple of days ago on this.