Jasper, do you know what species of so-called "parasites" have developed a symbiotic relationship with humans (hookworms is one, obviously, and it sounds like maybe tapeworms) and what species of animal(s) were their original host(s) before our ancestors first acquired them? In a traditional society, approximately what percentage of people will host parasite-like symbionts?
In a traditional society all members will host a varying number of parasites throughout their lives, starting in infancy. Protozoa from water sources, helminths from food and the soil, bacteria and molds and viruses from food, water and the soil.
Infections would be concurrent, multiple helminths at various points. When I got hookworm again, later on, from Central America, the village I surveyed (I obtained 50 stool samples), which was Ketchki Mayan, showed 90% had roundworm (Ascaris) and 25% had hookworm (Necator). None had tapeworm, but then only one villager kept cows and no beef was consumed in the village. Some undoubtedly had trichinosis, pork was raised and widely consumed (I got to see two slaughtered and butchered as it was a feast day one day I was there). But that would not show up in my stool survey.
So it would be with protozoa, all the water there was taken either from wells which were clearly infiltrated with river water, or directly from the river. Other villages were upstream and no doubt used that same river as water source, sewer, bath and plaything, just as the village I stayed in did.
The men all hunted and fished around the village, that would undoubtedly cause various infections or infestations.
Then there are various parasites of the skin, bed bugs, scabies, lice. Although they all slept in hammocks (instruments of torture, ouch), in other environments where bedding is used long term such parasites are common.
Athletes foot is an obvious parasite that is a fungus, but by one estimate even the impoverished intestines (in ecological terms) of westerners contain about 40,000 different species, numbering in the trillions, of different microorganisms.
the number of cells in or on your body are estimated to be 90% non-self, that is parasites. About sixty percent of your faeces by weight is bacteria, etc.
I would bet that a large proportion provide some benefit to us. But the science in this area is just beginning, so it will be hundreds of years before the relationships between us and all these organisms is characterised.
Also, what organisms benefit which individual varies tremendously. Only those who would otherwise develop immunological disorders without them have a mutualistic relationship with hookworm, to everyone else they are just parasites. At least so far as we know now.
As to original hosts, that is lost, although it could probably be found out with genetic research, say into all the species of hookworm. A lot came from living in groups, and hunting (eating raw meat for instance). Others came with agriculture and living in close proximity to animals, think swine and bird flu.
We probably originally carried many with us when we branched from our lessor evolved ancestors, too, all of which are gone of course.
There is even more to it than that but I am fading, it is late here.