The Little Animals by Sarah Tolmie

Antoni Van Leeuwenhoek, a quiet linen draper in Delft, has discovered a new world: the world of the little animals, or animalcules, that he sees through his simple microscopes. These tiny creatures are everywhere, even inside us. But who will believe him? Not his wife, not his neighbors, not his fellow merchants only his friend Reinier De Graaf, a medical doctor.