[Who saw them the first?].

From the theory of Girolamo Fracastoro in 1530, suggesting the participation of invisible seeds in the contagion of some diseases, to the universal genius Athanasius Kircher, who saw little worms in the blood of patients suffering from plague in 1659 and the final discovery of Anthony Van Leeuwenhoek in 1674, the existence of bacteria was surely in the mind of a few investigators. Kirchner, who seems to be the winner of this race, did not give any special meaning to his observations. Leeuwenhoek, instead, was deeply concerned about the importance of his discovery in the field of biology, but was unable to establish a link between these animalcula and human epidemic diseases.