The existence of viruses has been known ever since 1892 when the virus that causes the tobacco mosaic virus was isolated. Viruses are defined as organic entities made up of merely some genetic material and a protective protein coating. Alone, viruses are considered to be "dead" or inanimate, but when they are inside a cell, they are able to replicate in massive quantities. Although they had long been known, a virus's size or shape was not known until the 1940's with the development of the electron microscope. Most viruses known to this day are, in some way, harmful; all are parasites. Recently, though, a certain type of viruses, retroviruses, have shown potential to be beneficial, even life saving, to humans. Except for a few small differences retroviruses are basically is the same as any other virus. Like all viruses, retroviruses contain a core of genetic material, but retroviruses contain only RNA (ribonucleic acid) never DNA (deoxyribonucleic acid). (Retroviruses) Instead of injecting a new genome of DNA into the host cell, the retrovirus uses RNA as the blueprint for a new genome. Surrounding this core of RNA is a capsid protein. This serves as a sort of capsule to hold the core of the virus together. Along with the RNA inside the capsid is an enzyme called reverse transcriptase. This allows the virus to replicate once it has infected the host cell, transcribing viral RNA into a new genome of DNA. This entire core, contained in the capsid protein, is again encapsulated in a lipid envelope. The first line of protection against the environment is this envelope. Stuck in this envelope are the receptor-binding proteins. These proteins are essential for the function of the virus. A virus can only infect a cell if the receptor-binding proteins match the surface proteins of the potential host cell. Only a cell with the exact match of proteins can become a victim to a retrovirus. (Retropages) HIV-1, for example, has an...
