Cofty, have you heard of natural immunity? E.g. Europeans are naturally more resistant to HIV, whereas Africans are more prone to contract the disease. Some of those populations in endemic sleeping sickness areas, have become resistant against the parasite. So the potential is there. Through inoculation, one can "train" the immune system to recognize harmful antigens.
Another reason why humans are superior to animals. Because of our intelligence, we have become aware of these diseases and we can do something about it. Talking of parasites, there is a good reason why the Jews avoided pork. They definitely had an advantage over the nations. In a documentary the other day, they discussed Roman toilets. How people sat next to each other, and used communal spunges to wipe their backsides. Not a pleasent thought. Again the Jews were far ahead, washing hands, etc.