Which is the smallest of all microorganisms?

• A. Virus
• B. Bacteria
• C. Fungi
• D. Parasites

Answer: (A)

Size is the key factor. Viruses are the tiniest microorganisms because they are acellular particles made of genetic material wrapped in a protein shell (sometimes with a lipid envelope), and they range from about 20 nanometers up to a few hundred nanometers. Bacteria are single-celled organisms with their own metabolic machinery and cell structures, typically about 0.2 to 2 micrometers in size, which is larger than most viruses. Fungi, including yeasts and molds, are eukaryotic and generally even larger, often micrometers to tens of micrometers. Parasites cover a broad group, from single-celled protozoa that are usually larger than bacteria to multicellular worms that are much larger still. Because of their acellular, nanoscale size, viruses are the smallest among these microorganisms.