As a group, viruses are the smallest infectious agents. Their size places them in the realm of the ultramicroscopic. This term means that most of them are so minute ( < 0.2 μm) that an electron microscope is necessary to detect them or to examine their fine structure. They are dwarfed by their host cells: More than 2,000 bacterial viruses could fit into an average bacterial cell, and more than 50 million polioviruses could be accommodated by an average human cell. Animal viruses range in size from the small parvoviruses1 (around 20 nm in diameter) to megaviruses and pithoviruses that are as large as small bacteria (up to 750 nm in width) (figure 1). Some cylindrical viruses are relatively long (800 nm in length) but so narrow in diameter (15 nm) that they still cannot be seen without the high magnification and resolution of an electron microscope.
Figure 1 compares the sizes of several viruses with prokaryotic and eukaryotic cells and molecules.
Fig1. Size comparison of viruses with a red blood cell (eukaryotic) and various bacteria (prokaryotic). Viruses range from largest (1) to smallest (10). A molecule of a large protein (11) is included to indicate relative size of macromolecules.
Super-sized viruses, ranging from 400 µm to 1,500 µm across, are a recent phenomenon to scientists, so large that they were mistakenly viewed as small bacteria and ignored by virus hunters. Mimiviruses, pandorviruses, megaviruses (figure 2), and pithoviruses are 20 to 50 times larger than an average virus and even larger than small bacteria such as rickettsias. All three viruses were first discovered as parasites inside the cells of amoebas living in aquatic habitats. They have a typical viral cycle, multiplying in the host cytoplasm, and lack cellular structure and ribosomes, metabolic enzymes, and fission-style division. They also have many proteins that are unrelated to other viral groups, raising questions about where these unusual viruses fit on the “tree of life,” if any where at all.
Fig2. A monster virus—the megavirus. A TEM of one virus particle reveals its unique geometric shape, dark DNA core, and fine surface fibrils (4,500×). These viruses would be readily visible with an ordinary light microscope.
C. Abergel/IGS-IMM, CNRS-AMU
