They exist in every country of the world. In the last 40 years, the amount of international travel, of immigration and imported foods have taken parasites out of their localized homes and spread them throughout the world. We can get parasites by touch, sexual or otherwise, from family, friends, even pets. We can even get parasites from sharing a drink or breathing air in which parasitic eggs are carried on dust particles. Microscopic parasites can live in almost any organ of the body, including the liver, lungs, heart, bloodstream and even in your spinal cord and brain.
Parasites cause problems in a number of ways.
Realize that parasites, like most organisms need to eat, live to procreate, have to eliminate waste, and have survival mechanisms to insure that they can do those things. Some parasites cause damage because they eat actual cells in our bodies. Others cause damage because their growth interferes with the proper functioning of our organs, our joints, our nerves, etc. Dr. Thomas Brooks suggests that tapeworms have so adapted to living in the human body that we are totally without symptoms in their presence. Even though tapeworms don't directly cause disease, they're eating the nutrients that you're trying to live off.