<human>:  Antonie van Leeuwenhoek was a Dutch scientist. What did he discover?
<bot>: Antonie van Leeuwenhoek was the first person to discover microorganisms. He used a microscope to look at the tiny organisms that he found in water samples, and he discovered that these organisms were too small to be seen by the naked eye. He was the first person to discover bacteria, and he also discovered protozoa and fungi.