Like I said, the Amoeba is pretty rare and the body tolerates worms pretty well. We did evolve swimming and drinking ground water so that is not too surprising. I think a good portion of the population is probably walking around with one or more types of worm inside them without symptoms.
CDC - Parasites
Just 1 worm -
"Toxoplasmosis is considered to be a leading cause of death attributed to foodborne illness in the United States. More than 60 million men, women, and children in the U.S. carry the Toxoplasma parasite, but very few have symptoms because the immune system usually keeps the parasite from causing illness."
And with all the raw salads people eat, and the occasional sushi and raw oyster it would be amazing if year after year a worm or worm egg didn't go long for the ride. Hook worms just go into your foot when walking barefoot outside. Toxocara from dogs and cats, Chiagas from Wheel bugs (300,000 cases in the US, this killed millions in South America), Trichomoniasis (an STD infects 3.7 million in the US), tapeworm infection, also known as taeniasis, and cysticercosis occur globally.
Can't remember someone's name? Maybe a worm ate it
Like those watercress sandwiches? Liver fluke.
"Fasciola hepatica, which is also known as "the common liver fluke" or "the sheep liver fluke." A related parasite, Fasciola gigantica, also can infect people. Fascioliasis is found in all 5 continents, in over 50 countries, especially where sheep or cattle are reared. People usually become infected by eating raw watercress or other water plants contaminated with immature parasite larvae. The immature larval flukes migrate through the intestinal wall, the abdominal cavity, and the liver tissue, into the bile ducts, where they develop into mature adult flukes, which produce eggs. The pathology typically is most pronounced in the bile ducts and liver..."
Our modern medical system doesn't seem that well versed on worms. If they can't find the root of the problem, they will say you have a "syndrome" and send you home with a bill.