[About infection agents, zoophytes, animalcules and infusories]

By definition, zoophytes are organisms with characteristics which are intermediate between plants and animals. The concept is already outlined by Aristotle in his Historia Animalium. In the XVIII century, the great Swedish naturalist Carolus Linnaeus, in the tenth edition of his Systema Naturae, included the order Zoophyta within the class of Vermes. In this classification, for the first time the curious animalcules (infusories), discovered by van Leeuwenhoek in the late XVII century, were formally classified as zoophytes and were incorporated specifically into the genus Chaos. Audaciously, Linnaeus also conjectured that the infectious agents could be related to the animalcules-infusories, though he left the corresponding demonstration to posterity.