The word "parasites" is used in two senses. Everything living on you or in you, not just to perch, but to take its food from you is a parasite. No matter what its size, it can be called a parasite. But in some way the big worms need to be distinguished from the medium-sized amoebae, the even smaller bacteria and the smallest of all—viruses. So often the term parasite is reserved for the bigger things, from amoebae on up. In this book, the word parasite will be used in both ways as usual. You can easily guess what is meant. Parasitic worms are divided into roundworms and flatworms. Roundworms are round like earthworms even though they may be as thin as hairs (threadworms, filaria) or microscopically small (like Trichinella). Flatworms are more like leeches. They have a way to attach themselves sometimes with the head (scolex) like tapeworms, sometimes with a special sucker like flukes.
Worm parasites go through stages of development that can look very, very different from the adult.
Roundworms like Ascaris (common cat and dog roundworm), are simplest. The eggs are swallowed by licking or eating a bit of filth. They hatch into a tiny larva. The larva treks to the lungs. You cough it up and swallow it. Meanwhile it has molted a few times. It then crawls to the intestine where it becomes an adult, shedding eggs in your stool.
Worms usually have preferred locations. The favorite organ for Dirofilaria (dog heartworm) is the heart (even human heart). Sometimes the rules can be broken. My tests show Dirofilaria can live in other organs; too, if they are sufficiently polluted with solvents, metals and other toxins. Flatworms like tapeworms are much more complicated in their life history. You could eat the eggs accidentally with dirt. After hatching, the tiny larva burrows into its favorite organ. Your body encases it with a cyst. The white blood cells have been taught never to attack your body...and the cyst case is your body! So the tapeworm stage has safe residence for some time. If you are a meat eater, you could eat such a cyst if it happens to be lodged in the meat you are eating! Your teeth break it apart as you crunch. The little larva is swallowed and tries to attach itself to your intestine with its head. Then it grows longer by making segment after segment. The segments with their eggs leave with the bowel contents. I often see dog tapeworm of the small variety in their human family.
Flatworms like flukes are also very complicated. The eggs, passed out with bowel contents were not meant to be eaten as such. They were meant to hatch in a pond where snails and minnows eat them. The larva grows up in these new "secondary" hosts. Later, the snail sheds them and they attach themselves to foliage near the pond. They over-winter in a tough metacercarial cyst. An unsuspecting browsing animal now eats them. They come out of their metacercarial cyst as a small adult and quickly attach themselves to the intestine with a sucker. They now have "safe haven" and can go about maturing and laying eggs.
Four common flukes are: human intestinal fluke, human liver fluke, sheep liver fluke, and pancreatic fluke of cattle. Don't let the terms sheep and cattle mislead you. They are all found in humans.
(From: "The Cure for all Diseases", page 31; Copyright notice)