All forms of life are based on Ribonucleic acid (RNA), the extraordinary molecule that can both replicate genetic material and act as a catalyst (ribozyme) for many chemical processes in the cell. However, there are structures that contain RNA (or DNA) that are life-like in some important respects but are not quite life. The RNA molecule can shed fragments of itself, and these incomplete bits can approximate some kinds of life functions. But crucially, quasi-life forms are able to replicate themselves by pirating the machinery of living cells. Since they cannot survive independently, they are technically non-life structures.
Virus. The most familiar quasi-life form is the RNA fragment called a virus. This is a portion of genetic material from either RNA or DNA (but not both) with a minimal protein coat. The virus replicates itself by invading taking over the translation mechanism in living cells. Although they appropriate the machinery of living cells, not all viruses are dangerous. Some forms attack only bacteria, others select only plants, and others attack animals. Each virus selects one or more species, but never all. Flu epidemics and diseases such as AIDS have demonstrated that some viruses are able to jump from one species to another: from fowl or monkeys, for example, to humans. Because they are genetically minimal, viruses can mutate rapidly and resist destruction. How and when viruses evolved is unknown, but since RNA does shed fragments of itself it is reasonable to assume that viruses developed in tandem with living cells. Viruses are found in the genomes of all creatures and also in fossils.
Giant virus. The average virus has an average of 10 genes and a minimal protein shell, but there are a few giant viruses that have hundreds of genes and large protein shells. One example is called Mimivirus and another --the largest yet found-- is CroV. These viruses attack only single-cell organisms such as amoebas. Like other viruses, the giant forms can replicate only by commandeering the machinery of a living cell. But two key features of these viruses blur the distinction between life and nonlife. First, they have more genes than some bacteria; and second, they produce proteins. By definition, viruses are unable to produce proteins, so this property of the giant viruses suggests the possible need for a new category between life and quasi-life.
Viroid. Another type of RNA fragment comes from the enzymatic form of RNA, the ribozyme. The viroid is much smaller and much more limited than a virus, and contains only a shred of genetic information . It infects only plant cells, and replicates by taking over the host cell's transcription functions, a more complex process than that required by a virus.
Bacteriophage. A phage is a specialized virus that attacks only bacteria. There are millions of types, each associated with one or more specific bacterial hosts. These viruses may have considerable potential in medicine, as they attack deadly bacteria. Phages are also able to mutate as rapidly as the bacteria they infect, a great possible advantage since it is very hard to treat serious bacterial infections where constantly mutating bacteria render medications ineffective.
Prion. Proteinaceous infectious particles (shortened to prions) are deformed proteins accidentally produced by normal cells. These particles are not alive; they replicate the deformation by infecting other proteins in the brain in a kind of chain reaction. The destruction of brain cells in prion diseases causes irreversible brain wasting. The familiar prion diseases in livestock are scrapie, affecting sheep, and bovine spongiform encephalopathy (BSE) in cattle. Some prion diseases affect humans. An American physician, Daniel Gajdusek, proved that prion diseases could jump species (for this work he won the Nobel prize). The importance of this discovery was underscored by an outbreak of mad cow disease in cattle and its analogue in humans around 2009. The seriousness of the outbreak led public health authorities to slaughter millions of cattle. The major sources of prion infection in humans are rare meat and unsanitary slaughtering conditions, both of which can harbored prions. The public alarm in 2009 stimulated research, which eventually established that the prions could infect not only neural proteins, but blood and other human tissue.