Final answer:
Leeuwenhoek did not observe viruses, as they were too small to be seen with the technology available during his time.
Step-by-step explanation:
The type of microbe that was NOT observed by Leeuwenhoek is a virus. Antonie van Leeuwenhoek was the first to observe microorganisms, including algae, protozoans, and prokaryotes, using his handcrafted microscopes. However, due to their extremely small size and lack of technology at the time, viruses were not observed by Leeuwenhoek. Viruses are much smaller than bacteria and could not be seen with the microscopes available during his era. The study of viruses, known as virology, would take place much later after their discovery in the late 19th century.