Viruses are not considered alive. One quality of living things is that they have cells. Viruses do not have cells, rather they have a protein coat that protects their genetic material. They do not have any nuclei, organelles, or cytoplasm. Living things also reproduce. Viruses cannot reproduce by themselves and do not have the tools replicate their DNA, instead they inject their genetic material into a host cell and the host cell makes copies of the virus DNA. Unlike living things, viruses do not use energy, they become active when they come into contact with the host cell and then use the host cell’s energy to make more viruses. Viruses don’t need to consume energy to survive, they can survive on nothing. Viruses, unlike living things, also can not maintain homeostasis because they have no way to monitor the change in their environment. Living things grow. Viruses, however, are built in their fully formed state and never increase in size or complexity.

Viruses are thought to have emerged after cells because they need host cells to survive. However, some believe that viruses evolved before cells. They think that ancestors of viruses parasitized genetic material to make copies of themselves. Then this genetic material developed barriers to protect itself from viruses and evolved into cells. Some viruses may have evolved from plasmids and others from bacteria. Viruses facilitate horizontal gene transfer, which increases genetic diversity. Viruses have affected the human gene map, with eight percent of the genome being made up of endogenous retroviruses.Viruses have decreased population sizes, weeded out the less resistant hosts, and strengthened our immune system. New research shows that giant viruses may have spurred the development of cellular life. Giant viruses could have brought about evolution of DNA, the formation of the first cells, and life's split into three domains.