Perhaps it isn't luck at all??? after a half a million years of scavenging and predating upon ruminating animals our ancestors developed an innate immunity, to many of these parasites, much in the same way that the other apex predictors and scavengers like the big cats, wolves, vultures, hyenas and other such animals have managed too.
I am well aware of AVs theory of parasitic infection being beneficial to immune modulation and detoxification, but would like to broaden the scope to include the view that parasite host relationships are much more dynamic and integral to the health of the entire ecosystem, than is typically thought about. From the view that as is above so is below this host parasite paradigm must be expanded far beyond the world of the microscopic.
Grazing animals are in a way are symbiotic to the health and integrity of the living systems of terrestrial earth, yet if they become too numerous for the land to support, then the Gaian immune response is triggered. When in balance, the ruminating animals move freely and act as an exfoliator and fertilizer for the land, their bacterially active gut ecology terraforms foraged biomass into the perfect fertile substrate for new growth to utilize....yet when there is an imbalance and too many ruminators are present for the land to carry then the unseen intelligence goes to work to bring back balance. Simple grass mites, snales, or other waste devouring organisms which act to break down and reintegrate predigested biomass back into the soil, are carriers of these Gaian derived inoculation organisms....Ordinarily when possible, areas inoculated with heavy parasitic loads theses would be avoided by foraging animals who after getting one whiff of their own waste would quickly move on to greener pastures....These systems of symbiosis where parasites act as the earths own immune system, evolved in order to keep foraging animals from staying too long in one location so that the land could rejuvenate between being mowed down.....
Typically if for some reason an animal was unable or unwilling to move on to allow the land its period of rejuvenation then that animal would be inoculated with the worms, much in the same way the bodies immune system sends out antigens when inundated with invasive microbes. These inoculations in moderate situations cause the animal to move away from the infested area and seek out medicinal herbs and plants which would help stimulate the purging of the worms and the rebuilding of nutritional balance....but if the healing herbs are not available, and fresh pastures are not to be found then the parasite loads will build until the point the animal weakens, often becoming unable to reproduce or fend off predication....so the other layer of the earths immune system which is the higher order of predictors will go to work to thin out the numbers of the foraging parasites...giving the earths forage the time and space needed to regrow... after sufficient time has passed the eggs and parasitic cyst dissolve and the land becomes lush and open once again to the foragers. Its no coincidence that the amount of time needed for the land to rejuvenate closely correlates to the amount of time heavy parasite loads remain in the soil. This essence of symbiosis is dependent upon parasites and predication as equal partners that hold together interconnected web of life.
Understanding this helps us to see where modern cultivation has gone astray, by overburdening the land with more animals than it was ever designed to support, keeping those animals on the same plot for far too long, limiting access to wider varieties of forage and medicinal herbs, feeding those animals supplemental and processed foods, giving them drugs in a desperate attempt to cope with the imbalances, enabling the weak and unfit to survive generation after generation simply because of profit based motivations.... all of these things goes contrary to the natural order and will lead to compromises in the quality of the final product.