It's like people moving to better climates to make their living conditions more pleasant, or birds flying south for the winter. If the head remains, the entire worm will grow back. In order to understand how this is determined, you have to understand what a parasite does. A parasite eats, lays eggs and secretes. Depending on the kind, parasites will eat different things.
Some parasites love sugar, for instance.
If you are a person who craves sugar, you may have a sugar loving parasite. In fact, parasites are known to be one of the causes of diabetic tendencies and blood sugar discrepancies. These parasites live off the food that goes into your body. They exist mainly in the digestive tract, but can be found in the liver, as well as throughout the body. Other parasites actually get their nutrition directly from the cells of the body.