I came across some very interesting studies of a vaccine for fasciola flukes. If they present the antigen to the immune system before the person is exposed to the flukes. The body will automatically recognize it and fight against it. Some of the studies boasted close to a 70% success rate.
I think people who live in areas endemic to parasites naturally have more genetic defenses to attack them simply from the defenses that were built up and passed from generation to generation. So if you take the average Caucasian who has no defenses at all except the health of their immune system. The door is open for lots of problems to occur.