Many people are exposed to viruses and become sick, but for the majority of the population, they have no idea what a virus actually is. A virus is something that causes an illness and it is contagious. At its most basic level a virus is just a strand of DNA surrounded by a protein shell, which is what makes it different from other fragments of DNA such as prions and viroids. A virus cannot reproduce outside of a host cell and is considered by many scientists as not being truly 'alive'. Viruses can infect plants and animals and some are even able to infect bacteria.

Humans have dealt with viral diseases, such as rabies and smallpox, for many centuries. As early as the 18th century, people were observed who had been exposed to smallpox or cowpox, a similar virus and had subsequent immunity to smallpox. In the late 19th century a porcelain filter, developed by Charles Chamberland, was used to indirectly study the tobacco mosaic virus. Dmitri Ivanowski performed experiments on crushed tobacco leaf extracts that showed infected plants were still infectious after bacteria were filtered out. Many others performed experiments that showed similar filterable disease causing agents. Felix d'Herelle and Frederick Twort, working independently, found that bacteria could be infected by viruses, not just animals and plants. In the 1930s, Wendell Stanley showed that the tobacco mosaic virus was mostly protein and shortly after, it was further separated into genetic material and a protein shell.

A virus is not considered truly alive because it is missing several of the main qualities that scientists use to determine whether an organism is alive. The two biggest blocks to viruses being accepted as 'living' organisms is their lack of a cell membrane and their inability to metabolize on their own. Another criteria for living organisms that viruses lack is adaptation, or the ability to change and evolve depending on their environment. Viruses lack any hard parts that can fossilize such as bones and they are too small to leave imprints in sand, like some ancient jellyfish. Thus there are no fossil records of viruses, which makes showing relationships between different viruses very difficult.

Because viruses do not leave any fossil remains, the best way to hypothesize their origins has been to use molecular techniques, that is to study the genetic materials of different viruses and try to determine any relationships by finding similarities between them. There are two main hypotheses concerning virus origins. The first hypothesis, involving the small viruses with only a few genes, is that these small viruses may be runaway snippets of a living organisms DNA. These small DNA fragments could have come from plasmids or transposons, which are transferable genetic elements and are prone to entering or exiting genomes. The second hypothesis involves the larger viruses. These large viruses may have been small parasitic cells at one time and, through the process of reverse-evolution, lost any genes not needed by their parasitic lifestyle. There are a few bacteria, Rickettsia and Chlamydia, that support this hypothesis because they cannot reproduce outside of a host cell, like viruses.