Animalcule is a noun that refers to a microscopic animal, nearly or completely invisible to the naked eye. 

The Latin word ‘animalculum’ (ahn ee MAL kyoo loom) means ‘little animal.’ Since the 17th century, the word has been used — mostly in a scientific context — to describe animals too small to be seen without the aid of a microscope. Here’s an example:

The discovery of an anmalcule in the patient’s hair helped the scientists better understand the source of her illness. It’s fascinating to consider that had she developed the conditions in the years before microscope, her malady would have remained a mystery.