| Virology, often considered a part of | | | | molecular machinery traditionally |
| microbiology or of pathology, is the | | | | associated with bacteria. Is it a |
| study of biological viruses and virus | | | | simplified version of a parasitic |
| like agents: their structure and | | | | prokaryote, or did it originate as a |
| classification, their ways to infect and | | | | simpler virus that acquired genes from |
| exploit cells for virus reproduction, | | | | its host? |
| the diseases they cause, the techniques | | | | While viruses reproduce and evolve, they |
| to isolate and culture them, and their | | | | don't engage in metabolism and depend on |
| potential uses in research and therapy. | | | | a host cell for reproduction. The |
| A researcher in virology is a | | | | often-debated question of whether they |
| virologist. | | | | are alive or not is a matter of |
| Virus structure and classification | | | | definition that does not affect the |
| A major branch of virology is virus | | | | biological reality of viruses. |
| classification. Viruses can be | | | | Viral diseases and host defenses |
| classified according to the host cell | | | | One main motivation for the study of |
| they infect: animal viruses, plant | | | | viruses is the fact that they cause many |
| viruses, fungal viruses, and | | | | important infectious diseases, among |
| bacteriophages (viruses infecting | | | | them the common cold, influenza, rabies, |
| bacteria, which include the most complex | | | | measles, many forms of diarrhea, |
| viruses). Another classification uses | | | | hepatitis, yellow fever, polio, smallpox |
| the geometrical shape of their capsid | | | | and AIDS. Some viruses, known as |
| (often a helix or an icosahedron) or the | | | | oncoviruses, contribute to certain forms |
| virus's structure (e.g. presence or | | | | of cancer; the best studied example is |
| absence of a lipid envelope). Viruses | | | | the association between Human |
| range in size from about 30 nm to about | | | | papillomavirus and cervical cancer. Some |
| 450 nm, which means that most of them | | | | subviral particles also cause disease: |
| cannot be seen with light microscopes. | | | | Kuru and Creutzfeldt-Jakob disease are |
| The shape and structure of viruses can | | | | caused by prions, and hepatitis D is due |
| be studied with electron microscopy, | | | | to a satellite virus. |
| with NMR spectroscopy, and most | | | | The study of the manner in which viruses |
| importantly with X-ray crystallography. | | | | cause disease is viral pathogenesis. The |
| The most useful and most widely used | | | | degree to which a virus causes disease |
| classification system distinguishes | | | | is its virulence. |
| viruses according to the type of nucleic | | | | When the immune system of a vertebrate |
| acid they use as genetic material and | | | | encounters a virus, it produces specific |
| the viral replication method they employ | | | | antibodies which bind to the virus and |
| to coax host cells into producing more | | | | mark it for destruction. The presence of |
| viruses: | | | | these antibodies is often used to |
| DNA viruses (divided into | | | | determine whether a person has been |
| double-stranded DNA viruses and the much | | | | exposed to a given virus in the past, |
| less common single-stranded DNA | | | | with tests such as ELISA. Vaccinations |
| viruses), | | | | protect against viral diseases, in part, |
| RNA viruses (divided into positive-sense | | | | by eliciting the production of |
| single-stranded RNA viruses, | | | | antibodies. Specifically constructed |
| negative-sense single-stranded RNA | | | | monoclonal antibodies can also be used |
| viruses and the much less common | | | | to detect the presence of viruses, with |
| double-stranded RNA viruses), | | | | a technique called fluorescence |
| reverse transcribing viruses | | | | microscopy. |
| (double-stranded reverse-transcribing | | | | A second defense of vertebrates against |
| DNA viruses and single-stranded | | | | viruses, cell-mediated immunity, |
| reverse-transcribing RNA viruses | | | | involves immune cells known as T cells: |
| including retroviruses). | | | | the body's cells constantly display |
| In addition virologists also study | | | | short fragments of their proteins on the |
| subviral particles, infectious entities | | | | cell's surface, and if a T cell |
| even smaller than viruses: viroids | | | | recognizes a suspicious viral fragment |
| (naked circular RNA molecules infecting | | | | there, the host cell is destroyed and |
| plants), satellites (nucleic acid | | | | the virus-specific T-cells proliferate. |
| molecules with or without a capsid that | | | | This mechanism is jump-started by |
| require a helper virus for infection and | | | | certain vaccinations. |
| reproduction), and prions (proteins that | | | | RNA interference, an important cellular |
| can exist in a conformation which | | | | mechanism found in plants, animals and |
| induces other protein molecules to | | | | many other eukaryotes, most likely |
| assume that same conformation). | | | | evolved as a defense against viruses. An |
| The latest report by the International | | | | elaborate machinery of interacting |
| Committee on Taxonomy of Viruses (2005) | | | | enzymes detects double-stranded RNA |
| lists 5450 viruses, organized in over | | | | molecules (which occur as part of the |
| 2,000 species, 287 genera, 73 families | | | | life cycle of many viruses) and then |
| and 3 orders. | | | | proceeds to destroy all single-stranded |
| The taxa in virology are not necessarily | | | | versions of those detected RNA |
| monophyletic. In fact, the evolutionary | | | | molecules. |
| relationships of the various virus | | | | Every lethal viral disease presents a |
| groups remain unclear, and three | | | | paradox: killing its host is obviously |
| hypotheses regarding their origin exist: | | | | of no benefit to the virus, so how and |
| 1. Viruses arose from non-living matter, | | | | why did it evolve? Today it is believed |
| separately from and in parallel to other | | | | that most viruses are relatively benign |
| life forms, possibly in the form of | | | | in their natural host; the lethal viral |
| self-reproducing RNA ribozymes similar | | | | diseases are explained as resulting from |
| to viroids. | | | | an "accidental" jump of the virus from a |
| 2. Viruses arose from earlier, more | | | | species in which it is benign to a new |
| competent cellular life forms that | | | | one that is not accustomed to it (see |
| became parasites to host cells and | | | | zoonosis). For example, serious |
| subsequently lost most of their | | | | influenza viruses probably have pigs or |
| functionality; examples of such tiny | | | | birds as their natural host, and HIV is |
| parasitic prokaryotes are Mycoplasma and | | | | thought to derive from the benign monkey |
| Nanoarchaea. | | | | virus SIV. |
| 3. Viruses arose as parts of the genome | | | | While it has been possible to prevent |
| of cells, most likely transposons or | | | | (certain) viral diseases by vaccination |
| plasmids, that acquired the ability to | | | | for a long time, the development of |
| "break free" from the host cell and | | | | antiviral drugs to treat viral diseases |
| infect other cells. | | | | is a comparatively recent development. |
| It is of course possible that different | | | | The first such drug was interferon, a |
| alternatives apply to different virus | | | | substance that is naturally produced by |
| groups. | | | | certain immune cells when an infection |
| Of particular interest here is | | | | is detected, thus stimulating other |
| mimivirus, a giant virus that infects | | | | parts of the immune system. |
| amoebae and carries much of the | | | | |