There are a number of different properties that distinguish viruses from other organisms. They are extremely small, with a size range between 10 and 300 nanometers. This is about ten times smaller than bacteria. Viruses cannot survive on their own, and depend on hijacking the protein synthesis machinery of living cells to reproduce. Because of this, they are sometimes not considered true living things, but are instead called “organisms at the edge of life.” The domain name “Acytota” (meaning “without cells”) has been attributed to these organisms, but it does not receive wide use. Most scientists do not consider them to be alive.
Viruses are bits of genetic material, like a length of instruction tape, covered in a small protein shell called a capsid. Sometimes, they have very basic “appendages,” such as filaments or tail fibers, such as in bacteriophages (bacteria-killing viruses), but often, they are just a small package. Their shape may be helical, like a screw; icosahedral, like a geodesic dome; pleomorphic, like a little sponge; or resemble a bizarre spider robot out of science fiction, as in bacteriophages.
Instead of typical organisms, which reproduce via cell division, viruses reproduce at 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 may originate as rogue mobile genetic elements (transposons) from the genomes of bacteria, plants, or animals. This means that they may lack a conventional “family tree” that other organisms possess. Because these organisms do not fossilize well, studying their past can be very difficult. Examining them directly requires an expensive electron microscope.