Microbiology (from Greek μῑκρος, mīkros, "small"; βίος, bios, " life "; and -λογία, -logia ) is the study of microscopic organisms , either unicellular (single cell), multicellular (cell colony), or acellular (lacking cells).  Microbiology encompasses numerous sub-disciplines including virology , mycology , parasitology , and bacteriology .
Eukaryotic microorganisms possess membrane-bound cell organelles and include fungi and protists , whereas prokaryotic organisms—which all are microorganisms—are conventionally classified as lacking membrane-bound organelles and include eubacteria and archaebacteria . Microbiologists traditionally relied on culture, staining, and microscopy. However, less than 1% of the microorganisms present in common environments can be cultured in isolation using current means.  Microbiologists often rely on extraction or detection of nucleic acid , either DNA or RNA sequences.
Viruses have been variably classified as organisms,  as they have been considered either as very simple microorganisms or very complex molecules. Prions , never considered microorganisms, have been investigated by virologists, however, as the clinical effects traced to them were originally presumed due to chronic viral infections, and virologists took search—discovering "infectious proteins".