The answer is Antony van Leeuwenhoek.
Antony van Leeuwenhoek (1632 - 1723) was a Dutch scientist known as a father of microbiology. He used the microscope for seeing living things and even advance it. He was very interest in microbiology and protozoology so he studied pond water and peppercorn and hay infusion. In the pond water, he discovered many bacteria, protozoa (Vorticella and Volvox), and algae (Spirogyra). What is else known, he also discovered red blood cells in fish, birds, and humans.