Introduction
There are an estimated 8.7 million multicellular species of organisms on planet Earth (Mora et al., 2011). With roughly 1.2 million currently valid species, this means that 86% of existing species are still to be described. Basic classification is an indispensable step in understanding organisms and their modes of life. Therefore, scientists need to ensure technical terms are correct with universally accepted definitions. Throughout scientific history, many definitions have shifted. Examples are definitions of important terms in evolutionary biology, modes of life, and the nature of interactions, such as “symbiosis” (Leung and Poulin, 2008) and “microbe” (Hariharan, 2021). In this chapter, we will trace the remarkable history of parasitology and pathology in order to explore the definitions of parasite and pathogen through time.
Central to the exploration of parasitological and pathological history is the understanding that most people consider parasites and pathogens to be categorically negative. Approaching the subject through this lens has led to definitions that lack objectivity and limit a full understanding of the organism’s biology (Fig. 1; Tables 1, 2). This has caused misinterpretations and confusion and will be addressed in this chapter. Here, we regard parasites as multicellular organisms that live at the expense of a single host reducing its fitness but not directly causing death (sensu Haelewaters et al., 2017), and pathogens as microorganisms capable of producing disease under normal conditions of host resistance and rarely living in close association with the host without producing disease (sensu Onstad et al., 2006).
Parasitology throughout the literature
The original definition of parasite came from the Greek parasitos, “(someone) eating at another's table,” with para meaning beside and sitos meaning food or grain. The word was applied negatively to a person who gleaned meals from the wealthy in exchange for flattery, and later more generally for sycophants and toadies (Brooks and McLennan, 1993; Harant, 1995). Posidonius (c. 135 – c. 51 BCE), a Greek historian who traveled to Gallia, mentions that the Celtic chieftains had “parasites,” servants who followed them around and sing the chieftains’ praises during social gatherings (Tierney, 1959). In Coriolanus, a play by Shakespeare written in 1608, “parasite” is used to refer to obsequious courtiers.
Advances in microscopy starting in the 16th century enabled numerous observations of microscopic organisms living in and amongst the tissues of larger organisms. Knowledge of parasites in the biological sense appeared in the 17th century, first pertaining to parasitic plants such as mistletoe (Grew, 1681). This marked a shift from the term being used as a social descriptor of interactions between humans, to include non-human systems: “something feeding off another.” This scientific definition of parasite entered common parlance in the 18th century, but it was already imbued with a negative social tone. This interplay between social and scientific definitions of the word parasite means that a scientifically neutral term has a negative connotation for a general audience (Musolff, 2014).
Fossilized material including human feces has shown that early human ancestors were often infected with parasites (Camacho et al., 2018). The earliest known example of a human parasite was lung fluke (Paragonimus sp.) eggs found in fossilized feces in northern Chile. These fossils were estimated to be from 5900 BCE (Horne, 1985). Evidence of the malaria-causing protozoan parasite Plasmodium falciparum has been preserved in the famous mummy of the Egyptian pharaoh Tutankhamun (c. 1341 – c. 1323 BCE), which may have been the cause of his early death (Hawass et al., 2010). We can, however, not know for certain when the first parasites were observed as the oldest known script, the so-called cuneiform script, was only developed around 3400 BC (Schmandt-Besserat, 2015).
Parasites visible to the naked eye were described in ancient cultures such as Ancient Greece and Ancient Rome (Hoeppli, 1956), including descriptions by Aristotle (Hugo et al., 2001). According to the translation by Camus (1783), helminths were described as “[worms that] grow in the feces of animals, either before or after the feces have been released from their gut. These are three kinds, the flat worms, the round worms, and the ascarids. These kinds can particularly be found in the human body.” Urinary myiasis, an invasion of the urinary tract by dipteran larvae, had been recorded in ancient Rome by Plutarch, who described a small animal with the morphology of a Fannia larva in the sperm of a man (Hoeppli, 1956).
Having observed that ill effects often followed the consumption of pigs, both Islam and Judaism prohibit the consumption of pork. Scholars of these Abrahamic religions have connected these effects to what would eventually be understood as the muscle-infecting nematode, Trichinella (Neghina et al., 2012). This painful parasite can be transmitted through the consumption of undercooked meat, forming calcified cysts in mammalian muscles. For hundreds of years, the cause of this ailment was unknown, and the custom of pork prohibition became firmly entrenched in these religions.
In Medieval Europe, parasites were believed to be generated in “corrupt matter,” a process called spontaneous generation (Fig. 2). For instance, Gilbert Anglicus explained that worms in the gut arose when a patient developed an excess of phlegmatic humors (Henderson, 1918). He prescribed bitter, aromatic, or acid mixtures from plants to kill the worms and expel them by inducing diarrhea (Henderson, 1918).