A virus is a small infectious agent that can replicate only inside the living cells of other organisms. Viruses cause many different diseases in humans, animals, and plants. The best-known viruses include the human immunodeficiency virus (HIV), which causes AIDS, and influenza virus, which causes the flu.
Viruses are very diverse. They range in size from about 20 nanometers (nm) to 250 nm. Some viruses have a protein coat that protects their genetic material; others do not. The coat may be surrounded by an envelope of lipid molecules derived from the host cell membrane. Most enveloped viruses are much larger than non-enveloped viruses.
The genome of a virus may be composed of DNA or RNA, but not both. The vast majority of viruses have RNA genomes; however, some important human pathogens, such as HIV and hepatitis C virus (HCV), have DNA genomes. The largest known viral genome belongs to Mimivirus, with 1 million base pairs (bp). In contrast, the smallest known viral genome consists of just four genes encoding for proteins involved in replication within bacteriophages—viruses that infect bacteria—known as phage T4’s tail fibers subunit genes SS1–4 . These four genes occupy only 3 kb of genomic space!
The shapes of viruses are also quite varied and are classified into three main groups: spherical (or icosahedral), rod-shaped, and filamentous . Spherical viruses are by far the most common type; they typically have diameters between 30 and 200 nm (Figure 1). Examples include adenoviruses, which cause respiratory infections, and noroviruses, which cause gastroenteritis (stomach flu). Rod-shaped viruses are slightly less common; they typically measure 100–300 nm in length and 20 nm in diameter . An example is tobacco mosaic virus (TMV), which infects tobacco plants causing them to develop mosaic patterns on their leaves (Figure 2). Finally, filamentous viruses are long and thin—they can be over 10 micrometers (μm) in length but only about 8 nm wide ! An example is bacteriophage MS2; this phage infects enterobacteria such as Escherichia coli resulting in lysis—the bursting open—of bacterial cells upon infection (Figure 3).
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