The parasitic organisms described here can be divided into four categories. There are Protozoans, or single celled parasites; Platyhelminthes, otherwise known as flatworms; Nematodes, also known as roundworms; and Arthropods, or insects.

Parasites, by definition, depend upon and live inside another organism (called a host) for some portion of their lives. They use these hosts both as a source of food and as a place to lay their eggs. Some parasites use more than one host to complete their life cycles. For instance, the human lung fluke are carried by crabs during their larval, or cercaira stage. Then these crabs (called secondary hosts) are eaten and the lung flukes grow to maturity inside the unlucky eater (called the final host). The mature lung flukes lay their eggs inside these final hosts. The eggs are passed in the final host's feces. The feces enter the water supply, come into contact with crabs, and the whole cycle begins again.

Parasites are in most cases harmful, or at the very least not helpful, to their hosts. In humans, they cause or carry a variety of diseases, most of which may lead to death. You can protect yourself from many parasites by properly preparing your food and avoiding contaminated water. Of course, no one is completely safe, because there isn't much you can do about the parasites that are able to bore into your skin or enter your lungs when you inhale contaminated dust.