Parasites can be defined as organisms that live in or on another organism called a host. In most situations, the [Page 782]parasite benefits from this relationship, often at the expense of the host organism. Traditionally, parasites include protozoans and helminths. However, today, the term parasite is sometimes used to describe the multitude of viruses, bacteria, fungi, plants, and animals, including ticks, mites, and lice, that act in a parasitic fashion. Traditional parasites (protozoans and helminths) are responsible for many diseases in animals and humans and are transmitted to their host most often through the ingestion of contaminated food or water or arthropods, which act as intermediate hosts and vectors. Parasites pose health risks and economic costs in livestock and in humans and are often associated ...