One of the great geniuses of biology was the Swedish physician Carlos Linnaeus (1707-1778). He was called princeps botanicorum for his great contribution to the classification of plants. However, his undying fame is due to his work Systema Naturae in which he creates a binomial taxonomic system to classify all living and non-living beings into three kingdoms: the mineral kingdom, the plant kingdom and the animal kingdom. In his taxonomic scheme, the animalicles or microorganisms discovered by the Dutch scholar Antoine van Leeuwenhoek in 1676, were tentatively classified in the animal kingdom, within the class Vermes or Worms. The idea that these little animals were the cause of infectious diseases was imagined by Linnaeus and developed in depth by Johannes C. Nyander and Johannes Carolus Roos, two of his disciples, who published this idea in their theses Exanthemata viva in 1757 and Mundus invisibilis in 1767, respectively.