A virus is a small particle which can infect other biological organisms. Viruses are obligate intracellular parasites meaning that they can only reproduce by invading and taking over other cells as they lack the cellular machinery for self reproduction. The term virus usually refers to those particles which infect eukaryotes (multi-celled organisms and many single-celled organisms), whilst the term bacteriophage or phage is used to describe those infecting prokaryotes (bacteria and bacteria-like organisms).
Typically these particles carry a small amount of nucleic acid (either DNA or RNA) surrounded by some form of protective coat consisting of protein, or protein and lipid. Importantly a virus' genomes code not only for the proteins needed to package its genetic material but for those proteins needed by the virus to reproduce during its infective cycle.
The word comes from the Latin virus, referring to poison and other noxious things. Today it is used to describe the biological viruses discussed above and also as a metaphor for other parasitically-reproducing things, such as ideas. The term computer virus has become another well-defined sense of the word. The word virion or viron is used to refer to a single infective viral particle.
The English plural form of virus is viruses. No reputable dictionary gives any other form, including such "reconstructed" Latin plural forms as viri. (No plural form actually appears in any extant Latin manuscript). (See plural of virus).
Viruses: non-living or alive?
A virus makes use of existing enzymes and other molecules of a host cell to create more virus particles. Viruses are neither unicellular nor multicellular organisms; they are somewhere between being living and non-living. Viruses have genes and show inheritance, but are reliant on host cells to produce new generations of viruses. Many viruses have similarities to complex molecules. Like DNA, viruses undergo molecular replication and they can often be crystallized. Because viruses are dependent on host cells for their replication they are generally not classified as "living". Whether or not they are "alive", they are obligate parasites, and have no form which can reproduce independently of their host. Like most parasites they have a specific host range, sometimes specific to one species (or even limited cell types of one species) and sometimes more general.
Viruses form when molecules are assembled together to provide complex structures, and the self-assembly of viruses has implications for study of the origin of life. If the requirement for autonomous self-reproduction is abandoned, it can be argued that viruses are alive. Some small viruses are more efficient than most cellular life forms as their ratio of functions to working parts is so high. There is no rule in any 'book of life' that prohibits leveraging the components and services of other systems. If viruses are alive then the prospect of creating artificial life is enhanced or at least the standards required to call something artificially alive are reduced.
Viruses as tools for exploring basic cellular processes
Viruses are important to the study of molecular and cellular biology because they provide simple systems that can be used to manipulate and investigate the functions of many cell types. Below, we discuss how viral replication depends on the metabolism of the infected cell. Therefore, the study of viruses can provide fundamental information about aspects of cell biology and metabolism. The rapid growth and small genome size of bacteria make them excellent tools for experiments in biology. Bacterial viruses have also further simplified the study of bacterial genetics and have deepened our understanding of the basic mechanisms of molecular genetics. Because of the complexity of an animal cell genome, viruses have been even more important in studies of animal cells than in studies of bacteria. Numerous studies have demonstrated the utility of animal viruses as probes for investigating different activities of eukaryotic cells. Other examples in which animal viruses have provided important models for biological research of their host cells include studies of DNA replication, transcription, RNA processing, and protein transport.
One family of animal viruses, the retroviruses, contains RNA genomes in their virus particles but synthesize a DNA copy of their genome in infected cells. Retroviruses provide an excellent example of how viruses can play an important role as models for biological research. Studies of these viruses are what first demonstrated the synthesis of DNA from RNA templates, a fundamental mode for transferring genetic material that occurs in both eukaryotes and prokaryotes.
Viruses as tools for genetic engineering
Geneticists regularly use viruses to introduce DNA into cells that they are studying. Attempts to treat human diseases through genetic engineering have also made use of viruses.
Viral structure and anatomy
Viruses typically consist of a protein coat (the envelope), a protein core (the capsid) that encloses the viral genes and the viral genetic material itself. The envelope, normally derived from the cell membrane of the previous host, protects the viral genome contained within and also provides the mechanism by which the virus infects its host.
Some viruses are quite large, particularly some that exist as metabolic parasites inside host cells. A giant intracellular virus, Mimivirus, survives inside amoebae that can be found in the water of cooling towers. Mimivirus has a DNA genome of 1181404 base pairs, larger than the genomes of several bacteria.
Because viruses are acellular and do not use ATP, they must utilize the machinery and metabolism of the host cell to reproduce. For this reason, viruses are called obligate intracellular parasites. Before a virus has entered a host cell, it is called a virion — a package of viral genetic material. Virions can be passed from host to host either through direct contact or through a vector, or carrier. Inside the organism, the virus can enter a cell in various ways. Bacteriophages—bacterial viruses—attach to the cell wall surface in specific places. Once attached, enzymes make a small hole in the cell wall, and the virus injects its DNA into the cell. Other viruses (such as HIV) enter the host via endocytosis, the process whereby cells take in material from the external environment. After entering the cell, the virus's genetic material begins the destructive process of taking over the cell and forcing it to produce new viruses.
There are three different ways genetic information contained in a viral genome can be reproduced. The form of genetic material contained in the viral capsid, the protein coat that surrounds the nucleic acid, determines the exact replication process. Some viruses have DNA, which once inside the host cell is replicated by the host along with its own DNA. Then, there are two different replication processes for viruses containing RNA. In the first process, the viral RNA is directly copied using an enzyme called RNA replicase. This enzyme then uses that RNA copy as a template to make hundreds of duplicates of the original RNA. A second group of RNA-containing viruses, called the retroviruses, uses the enzyme reverse transcriptase to synthesize a complementary strand of DNA so that the virus's genetic information is contained in a molecule of DNA rather than RNA. The viral DNA can then be further replicated using the resources of the host cell.
Steps associated with viral reproduction
- Attachment, sometimes called absorption: The virus attaches to receptors on the host cell wall.
- Penetration: The nucleic acid of the virus moves through the plasma membrane and into the cytoplasm of the host cell. The capsid of a phage, a bacterial virus, remains on the outside. In contrast, many viruses that infect animal cells enter the host cell intact.
- Replication: The viral genome contains all the information necessary to produce new viruses. Once inside the host cell, the virus induces the host cell to synthesize the necessary components for its replication.
- Assembly: The newly synthesized viral components are assembled into new viruses.
- Release: Assembled viruses are released from the cell and can now infect other cells, and the process begins again.
When the virus has taken over the cell, it immediately directs the host to begin manufacturing the proteins necessary for virus reproduction. The host produces three kinds of proteins: early proteins, enzymes used in nucleic acid replication; late proteins, proteins used to construct the virus coat; and lytic proteins, enzymes used to break open the cell for viral exit. The final viral product is assembled spontaneously, that is, the parts are made separately by the host and are joined together by chance. This self-assembly is often aided by molecular chaperones, or proteins made by the host that help the capsid parts come together.
The new viruses then leave the cell either by exocytosis or by lysis. Envelope-bound animal viruses instruct the host's endoplasmic reticulum to make certain proteins, called glycoproteins, which then collect in clumps along the cell membrane. The virus is then discharged from the cell at these exit sites, referred to as exocytosis. On the other hand, bacteriophages must break open, or lyse, the cell to exit. To do this, the phages have a gene that codes for an enzyme called lysozyme. This enzyme breaks down the cell wall, causing the cell to swell and burst. The new viruses are released into the environment, killing the host cell in the process.
The origin of viruses is not entirely clear, but the currently favoured explanation is that they are derived from their host organisms, originating from transferrable elements like plasmids or transposons. It has also been suggested that they may represent extremely reduced microbes, appeared separately in the primordial soup that gave rise to the first cells, or that the different sorts of viruses appeared through different mechanisms.
Human viral diseases
Examples of diseases caused by viruses include the common cold, which is caused by any one of a variety of related viruses; smallpox; AIDS, which is caused by HIV; and cold sores, which are caused by herpes simplex. Recently it has been shown that cervical cancer is caused at least partly by papillomavirus (which causes papillomas, or warts), representing the first significant evidence in humans for a link between cancer and an infective agent. There is current controversy over whether borna virus, previously thought of primarily as the causative agent of neurological disease in horses, could be responsible for psychiatric illness in humans. The relative ability of viruses to cause disease is described in terms of virulence.
The ability of viruses to cause devastating epidemics in human societies has led to concern that viruses will be weaponized for biological warfare. Further concern was raised by the successful recreation of a virus in a laboratory. Much concern revolves around the smallpox virus, which has devastated numerous societies throughout history, and today is extinct in the wild. In fact, smallpox has been used in a crude form of biological warfare by British colonists against a tribe of Native Americans.
This episode of biological warfare was part of a larger phenomenon of Native American populations being devastated by contagious diseases, particularly smallpox, brought to the Americas by European colonists. It is unclear how many Native Americans were killed by smallpox after the arrival of Columbus in the Americas, but it may have been very large. The damage done by this disease may have significantly aided European attempts to displace or conquer the native population. Jared Diamond argued in his book Guns, Germs, and Steel that highly contagious diseases develop in agricultural societies and regularly aid those societies when they expand into the territories of non-agricultural peoples.
Laboratory diagnosis of pathogenic viruses
Detection and subsequent isolation of viruses from patients are a very specialised laboratory subject. It is normally not done in both clinics and small hospitals. Usually, it is the responsibility of government to set up national public health laboratory and perform virus-related laboratory diagnosis.
Prevention and treatment of viral diseases
Because they use the machinery of their host cells, viruses are difficult to kill. The most effective medical approaches to viral diseases, thus far, are vaccination to provide resistance to infection, and drugs that treat the symptoms of viral infections. Patients often ask for antibiotics, which are useless against viruses, and their misuse against viral infections is one of the causes of antibiotic resistance in bacteria. That said, sometimes the prudent course of action is to begin a course of antibiotic treatment while waiting for test results to determine whether the patient's symptoms are caused by a virus or a bacterial infection.