Historical definitions of pathogens

The earliest recorded epidemic occurred during 430 BCE amidst the Peloponnesian War (Retief and Cilliers, 1998). This disease, suspected to have been typhoid fever, spread through Libya, Ethiopia, and Egypt, to eventually hit both the Athenians as well as the Spartans, and an estimated two-thirds of the population died (Retief and Cilliers, 1998; Littman, 2009). During this time, the miasma theory (advanced by Hippocrates of Kos) was widely accepted as an explanation for diseases. The theory posited that a nebulous “miasma” emerged from rotting organic materials and made people ill.
The basic tenets of Germ theory—the idea that growth and reproduction of microorganisms can cause disease—was first reported by Islamic physicians in the Islamic Golden Age. In his famous work Canon of Medicine (1025), Abu Ali Sina (also known as Ibn Sina or Avicenna) claimed that a quarantine (a period of 40 days) was essential to prevent further spread of contagious infections (Whinder, 2012). Over 500 years later, European scientists started to arrive at similar conclusions. In his 1546 work De Contagione et Contagiosis Morbis [On Contagion and Contagious Diseases], Girolamo Fracastoro suggested that infections could result from tiny, self-multiplying “bodies.” These bodies could be spread by direct or indirect contact through infected objects or could even be passed through the air over long distances. In 1762, physician Marcus von Plenciz hypothesized that each disease was caused by a different organism dwelling within the human body—a hypothesis later proven by, amongst others, Robert Koch.