What is the scientific field that studies microscopic organisms?

The scientific field that studies microscopic organisms is called microbiology.

Microbiology examines microorganisms and other microscopic biological entities, including bacteria, archaea, fungi, protists, and many microscopic algae. Its major branches include bacteriology, mycology, virology, parasitology, microbial genetics, and microbial ecology. Microbiologists study how these entities live, reproduce, evolve, cause disease, and affect ecosystems or industry.

The field’s history began with improved microscopes. Robert Hooke described mold structures in 1666, while Antonie van Leeuwenhoek observed bacteria and other tiny organisms in the 1670s using microscopes of his own design. During the 19th century, Louis Pasteur and Robert Koch helped establish microbiology as an experimental science and connected particular microorganisms with fermentation and disease.

Microbiology overlaps with neighboring fields but is not identical to them. Immunology focuses on immune defenses, pathology studies disease and its effects on tissues, and epidemiology tracks disease patterns through populations. Viruses are often treated cautiously because many biologists do not classify them as living organisms, yet virology remains an important subfield of microbiology.