Following the invention of microscopes, Francesco Redi (1668) described external parasites for the first time. Amongst others, he described nasal flies of deer (Cephenemyiinae larvae) and the sheep liver fluke (Fasciola hepatica). One of his most significant contributions was his contradiction to the spontaneous generation theory. Redi tested the spontaneous creation of maggots via fresh meat in two jars; one jar was left open, while the other jar was closed with a cloth. His experiment demonstrated that maggots do not emerge from meat, but instead that maggots came from fly eggs. In 1681, Antonie van Leeuwenhoek, the founder of microbiology, wrote in a letter to Robert Hooke that he had found “animalcules” in his stool. These cells were most likely Giardia lamblia trophozoites (Feely et al., 1984).