A virus is an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria.

Viruses are microscopic; they range in size from about 20 to 400 nanometres in diameter (1 nanometre = 10-9 meters). By contrast, the smallest bacteria are about 400 nanometres in size. A virus consists of a single- or double-stranded nucleic acid and at least one protein surrounded by a protein shell, called a capsid; some viruses also have an outer envelope composed of fatty materials (lipids) and proteins. The nucleic acid carries the virus's genome--its collection of genes--and may consist of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The protein capsid provides protection for the nucleic acid and may contain enzymes that enable the virus to enter its appropriate host cell. Some viruses are rod-shaped, others are icosahedral (a roughly spherical shape that is actually a 20-sided polygon), and still others have complex shapes consisting of a multisided "head" and a cylindrical "tail."

Viruses are classified on the basis of their nucleic acid content, their size, the shape of the capsid, and the presence of a lipoprotein envelope. Thus, the primary division is into two classes: RNA viruses and DNA viruses.

Outside of a living cell, a virus is a dormant particle; but within an appropriate host cell, it becomes an active entity capable of subverting the cell's metabolic machinery for the production of new virus particles.

The virus's developmental cycle begins with the entrance of the particle's nucleic acid, and in some cases its proteins, into a susceptible host cell. Bacterial viruses adsorb and firmly attach to the surface of the bacterium and then penetrate the rigid cell wall, transmitting the viral nucleic acid into the host. Animal viruses enter host cells by a process called endocytosis. Plant viruses, by contrast, enter through wounds in the cell's outer coverings--e.g., through abrasions made by wind or through punctures made by insects.

Once inside the host cell, the viral genome usually directs the production of new viral components--new viral protein is synthesized and new viral nucleic acid is produced. These components are then assembled into complete virions (entire virus particles containing nucleic acid enclosed within a protein capsule), which are discharged from the host cell.

Among bacterial viruses, called bacteriophages, or phages, the release of the new virions is accomplished by lysing (bursting) the host cell. This pattern is called a lytic type of infection. Bacteriophages sometimes, however, show a different pattern of infection, called the lysogenic, or temperate, type. In a lysogenic infection, the viral genome is integrated into the chromosome of the host cell and becomes known as a prophage, which replicates in concert with that chromosome prior to cell division. In such cases, no progeny virions are produced and the host cell remains intact. The viral genome, however, is being passed on to each new generation of cells that stem from the original host. At some point, the prophage can be excised from the host cell's genome, usually owing to an environmental trigger such as ultraviolet radiation. The viral genome is then able to replicate, with the subsequent bursting of the host cell and the release of new virions.

Occasionally during the prophage's exit, some of the host cell's genetic material will be removed as well. If this phage subsequently infects another host bacterium, the piece of DNA from the previous host may become part of the new host's genome. This exchange of genetic information is called transduction, and the phage capable of carrying out the process is called a transducing phage.

Viral infections of plant and animal cells resemble those of bacterial cells in many ways. The release of new virions from plant and animal cells does not, however, always involve the lysing of the host cell as it does in bacteria. Particularly among animal viruses, the new virions may be released by budding off from the cell membrane, a process that is not necessarily lethal to the host cell.

In general, a viral infection produces one of four effects in a plant or animal cell: inapparent effect, in which the virus lives dormantly in the host cell; cytopathic effect, in which the cell dies; hyperplastic effect, in which the cell is stimulated to divide prior to its death; and cell transformation, in which the cell is stimulated to divide, take on abnormal growth patterns, and become cancerous.

Viral infections in animals can be either localized or disseminated to many distant locations in the body. Some animal viruses produce latent infections; in these the virus persists in a quiescent state, becoming periodically active in acute episodes, as in the case of the herpes simplex viruses.

There are a number of different ways an animal can respond to a viral infection. Fever is a general response; many viruses are inactivated at temperatures just slightly above the host's normal body temperature. The secretion of interferon by infected animal cells is another general response. Interferon stimulates infected cells and those close by to produce proteins that interfere with virus replication. Humans and other vertebrates also can mount an immunological attack against a specific virus. The immune system produces antibodies and sensitized cells that are tailor-made to neutralize the infecting virus. These immune defenders circulate through the body long after the virus has been neutralized, thereby providing long-term protection against reinfection by this virus. This long-term immunity is the basis for active immunization against viral diseases. In active immunization, a weakened or inactivated strain of an infectious virus is introduced into the body. This virus does not provoke an active disease state, but it does stimulate the production of immune cells and antibodies, which then protect against subsequent infection by the virulent form of the virus. Active immunizations are now routine for such viral diseases as measles, mumps, poliomyelitis, and rubella.

In contrast, passive immunization is the injection of antibodies from the serum of an individual who has already been exposed to the virus. Passive immunization is used to give short-term protection to individuals who have been exposed to such viral diseases as measles and hepatitis. It is useful only if provided soon after exposure, before the virus has become widely disseminated in the body.

The treatment of an established viral infection usually is restricted to palliation of the specific symptoms; for example, fluid therapy may be used to control dehydration, or aspirin may be given to relieve aches and reduce fever. There are few drugs that can be used to directly combat an infecting virus. This is because viruses use the machinery of living cells for replication; drugs that inhibit viral development also inhibit the functions of the host cell. Nonetheless, a small number of antiviral drugs are available for specific infections.

The most successful controls over viral diseases are epidemiological. Large-scale active immunization programs, for example, can break the chain of transmission of a viral disease. Worldwide immunization is credited with the eradication of smallpox, once one of the most feared viral diseases. Because many viruses are carried from host to host by insects or contaminated food, insect control and hygienic food handling can help eliminate a virus from specific populations.

Historical descriptions of viral diseases date as far back as the 10th century BC. The concept of the virus, however, was not established until the last decade of the 19th century, when several researchers obtained evidence that agents far smaller than bacteria were capable of causing infectious diseases. The existence of viruses was proved when bacteriophages were independently discovered by researchers in 1915 and 1917.

Because their genomes are small and because large quantities can be prepared in the laboratory, bacteriophages are a favourite research tool of molecular biologists. Studies of bacteriophages have helped to illuminate such basic biological processes as genetic recombination, nucleic acid replication, and protein synthesis.

Excerpt from the Encyclopedia Britannica without permission.