Viruses are extremely small. While bacteria typically have a size ranging between 0.5 - 5.0 micrometers, viruses are around ten times smaller, with a size range between 10 and 300 nanometers. Viruses cannot survive on their own, and depend upon hijacking the protein synthesis machinery of living cells to reproduce. Because of this, viruses are sometimes not considered true living things, but are rather called "organisms at the edge of life." The domain name "Acytota" (meaning "without cells") has been attributed to viruses, but it does not receive wide use. Most scientists do not regard viruses as living.

Viruses are bits of genetic material, like a length of instruction tape, covered in a small protein shell called a capsid. Sometimes viruses have very basic "appendages," such as filaments or tail fibers, such as in bacteriophages (bacteria-killing viruses), but oftentimes are just a small package. Their morphology may be helical, like a screw, icosahedral, like a geodesic dome, pleomorphic, like a little sponge, or resembling a bizarre spider robot out of science fiction, as in bacteriophages.

Instead of typical organisms, which reproduce via cell division, viruses reproduce as a hyper-exponential rate by infiltrating cells and using their protein synthesis machinery to pump out copies of the virus. In just ten minutes, a virus may take over a cell, copy itself hundreds of times, and kill the cell. Some viruses have a calculated replication time of about 70 seconds. By comparison, the fastest bacterial replicators only double their biomass every 20 minutes or so.

Viruses do not evolve or develop in ways similar to conventional living things. They do mutate and evolve, but some viruses may originate as rogue mobile genetic elements (transposons) from the genomes of bacteria, plants, or animals. This means that viruses may lack a conventional "family tree" that other organisms possess. Because viruses do not fossilize well, studying their past can be very difficult. Examining viruses directly requires an expensive electron microscope.