A virus is a rudimentary biosystem that has some of the aspects of a living system such as having a genome (genetic code) and the ability to adapt to its environment. A virus, however, cannot acquire and store energy and is therefore not functional outside of its hosts. Viruses and retroviruses infect cells by attaching themselves to the host cell and either entering themselves or injecting their genetic material into the cell and then reproducing its genetic material within the host cell. The reproduced virus then is released to find and attack more host cells. The difference between a virus and retrovirus is a function of how each replicates its genetic material. A virus has a single strand of genetic material—either DNA or RNA. A retrovirus consists of a single strand of RNA. Once a retrovirus enters a cell, it collects nucleotides and assembles itself as a double strand of DNA that splices itself into the host’s genetic material. Retroviruses were first identified by David Baltimore (1938–) and Howard Temin (1934–). They were awarded the Nobel Prize in Physiology or Medicine in 1975 for their discovery.