When a parasitologist refers to “parasites” they do not mean an evolutionary related grouping of organisms, rather it is a descriptive term that (as far as I can tell) has clear members and others, that by convention, are not included. This is my working definition of the term “parasite” that attempts to reflect this convention:  1) They are eukaryotic organisms and hence are not viruses, bacteria or prions, 2) they depend upon at least one living host for their survival and this relationship is not mutually beneficial, in other words, some type of harm is inflicted to the parasitized host, and finally, 3) the parasite alternates between at least two different environments (host – outside; host – insect; host – host of the same or different species) and as such, uses developmental programs to change its form to enter into or survive within the various environments it encounters. Transmission is key to a parasite’s life cycle, which ensures the parasite will receive the desired nutrients at the various places it goes. Parasites can be large and multi-cellular (helminths) or microscopic single celled organisms (protozoans), they can infect the outside surface of a host or live within. They can inflict great pain, they can produce mild infections. A parasite may have a strict species requirement for infection, others may be more promiscuous (i.e. “broad host range”). The variations on the theme of parasitism are many and is the topic of study for the over 30 parasites covered in Human Parasitology BIO123. Scrutinize the term “parasite” and you may wonder whether meat eating carnivores, Naegleria fowleri (presumably does not need a host for it’s survival) or barnacles attached to a whale (increasing drag during the whale’s swim) are parasites. Certainly there is a blurry boundary at the border of what constitutes a “true” parasite. Most medically important parasites would certainly fit the working definition above (Trichomonas vaginalis may be an exception as it doesn’t change forms to ensure transmission).