<human>:  Antonie van Leeuwenhoek was a Dutch scientist. What did he discover?
<bot>: Antonie van Leeuwenhoek was the first person to discover microorganisms. He was a microscopist who made discoveries in biology and medicine, and was the first person to describe microorganisms in the late 1600s. His observations led him to believe that microorganisms existed and were responsible for diseases.