Any of a heterogeneous class of agents that share three characteristics: (1) They consist of a nucleic acid genome surrounded by a protective protein shell, which may itself be enclosed within an envelope that includes a membrane; (2) they multiply only inside living cells, and are absolutely dependent on the host cells' synthetic and energy-yielding apparatus; (3) the initial step in multiplication is the physical separation of the viral genome from its protective shell, a process known as uncoating, which differentiates viruses from all other obligatorily intracellular parasites. In essence, viruses are nucleic acid molecules, that is, genomes that can enter cells, replicate in them, and encode proteins capable of forming protective shells around them. Terms such as “organism” and “living” are not applicable to viruses. It is preferable to refer to them as functionally active or inactive rather than living or dead.

The primary significance of viruses lies in two areas. First, viruses destroy or modify the cells in which they multiply; they are potential pathogens capable of causing disease. Many of the most important diseases that afflict humankind, including rabies, smallpox, poliomyelitis, hepatitis, influenza, the common cold, measles, mumps, chickenpox, herpes, rubella, hemorrhagic fevers, and the acquired immunodeficiency syndrome (AIDS) are caused by viruses. Viruses also cause diseases in livestock and plants that are of great economic importance. Acquired immune deficiency syndrome (AIDS) Plant pathology

Second, viruses provide the simplest model systems for many basic problems in biology. Their genomes are often no more than one-millionth the size of, for example, the human genome; yet the principles that govern the behavior of viral genes are the same as those that control the behavior of human genes. Viruses thus afford unrivaled opportunities for studying mechanisms that control the replication and expression of genetic material.

Although viruses differ widely in shape and size , they are constructed according to certain common principles. Basically, viruses consist of nucleic acid and protein. The nucleic acid is the genome which contains the information necessary for virus multiplication and survival, the protein is arranged around the genome in the form of a layer or shell that is termed the capsid, and the structure consisting of shell plus nucleic acid is the nucleocapsid. Some viruses are naked nucleocapsids. In others, the nucleocapsid is surrounded by a lipid bilayer to the outside of which “spikes” composed of glycoproteins are attached; this is termed the envelope. The complete virus particle is known as the virion, a term that denotes both intactness of structure and the property of infectiousness.

Viral genomes are astonishingly diverse. Some are DNA, others RNA; some are double-stranded, others single-stranded; some are linear, others circular; some have plus polarity, other minus (or negative) polarity; some consist of one molecule, others of several (up to 12). They range from 3000 to 280,000 base pairs if double-stranded, and from 5000 to 27,000 nucleotides if single-stranded.

Viral genomes encode three types of genetic information. First, they encode the structural proteins of virus particles. Second, most viruses encode enzymes capable of transcribing their genomes into messenger RNA molecules that are then translated by host-cell ribosomes, as well as nucleic acid polymerases capable of replicating their genomes; many viruses also encode nonstructural proteins with catalytic and other functions necessary for virus particle maturation and morphogenesis. Third, many viruses encode proteins that interact with components of host-cell defense mechanisms against invading infectious agents. The more successful these proteins are in neutralizing these defenses, the more virulent viruses are.

The two most commonly observed virus-cell interactions are the lytic interaction, which results in virus multiplication and lysis of the host cell; and the transforming interaction, which results in the integration of the viral genome into the host genome and the permanent transformation or alteration of the host cell with respect to morphology, growth habit, and the manner in which it interacts with other cells. Transformed animal and plant cells are also capable of multiplying; they often grow into tumors, and the viruses that cause such transformation are known as tumor viruses. Cancer (medicine) Oncology Retrovirus Tumor viruses

There is little that can be done to interfere with the growth of viruses, since they multiply within cells, using the cells' synthetic capabilities. The process, interruption of which has met with the most success in preventing virus multiplication, is the replication of viral genomes, which is almost always carried out by virus-encoded enzymes that do not exist in uninfected cells and are therefore excellent targets for antiviral chemotherapy. Another viral function that has been targeted is the cleavage of polyproteins, precursors of structural proteins, to their functional components by virus-encoded proteases; this strategy is being used with some success in AIDS patients. Cytomegalovirus infection Herpes Influenza Respiratory syncytial virus

Antiviral agents on which much interest is focused are the interferons. Interferons are cytokines or lymphokines that regulate cellular genes concerned with cell division and the functioning of the immune system. Their formation is strongly induced by virus infection; they provide the first line of defense against viral infections until antibodies begin to form. Interferons interfere with the multiplication of viruses by preventing the translation of early viral messenger RNAs. As a result, viral capsid proteins cannot be formed and no viral progeny results.

By far the most effective means of preventing viral diseases is by means of vaccines. There are two types of antiviral vaccines, inactivated virus vaccines and attenuated active virus vaccines. Most of the antiviral vaccines currently in use are of the latter kind. The principle of antiviral vaccines is that inactivated virulent or active attenuated virus particles cause the formation of antibodies that neutralize a virulent virus when it invades the body. Animal virus Plant viruses and viroids