also filterable viruses, or ultraviruses, agents of infectious diseases in plants, animals, and humans, which multiply only in living cells. Viruses are smaller than a majority of known microbes; practically all viruses can pass through bacterial filters. Unlike bacteria, viruses cannot be cultivated in conventional nutrient media. For experimental and medical purposes (such as preparing vaccines) viruses are cultivated in animals and plants, chick embryos, and in tissue and cell cultures. Viruses cause many diseases, such as smallpox, measles, influenza, poliomyelitis, plague in cattle and poultry, rabies, a number offish and amphibian diseases, silkworm jaundice, tobacco mosaic disease, oat pupation disease, and numerous diseases of fungi and blue-green algae. The bacteriophages are a vast group of viruses that infect bacteria.

The existence of infectious agents that pass through bacterial filters was first demonstrated in 1892 by D. I. Ivanovskii, who discovered the filterability of the causative agent of tobacco mosaic disease. Soon it was shown that other agents could pass through a filter, such as the causative agent of foot-and-mouth disease (1897), cattle plague (1899), fowl pox (1902), and rabies (1903). The word “virus” was first used in the present sense by M. Beijerinck (1899). Before that the word was sometimes also used for infectious microbial agents, such as Mycobacterium tuberculosis. With continuing studies, our knowledge about viruses is becoming more exact and precise. The causative agents of a number of diseases that were formerly included among the viruses, for example, rickettsias and the infectious agent of psittacosis, have been excluded from this group of organisms. Mature virus particles—virions, or virospores—are able to survive unfavorable conditions of the external environment and in this form do not show any signs of life. Following the entry into the organism, they infect susceptible cells and start to multiply and develop into mature daughter virus particles.

The structure and composition of virus particles. The shapes of virions are extremely varied. In many bacteriophages the shape is a head and a tail; the smallpox virus is rectangular in shape; the herpes and flu viruses are spherical; the tobacco mosaic disease virus is rod-shaped; the potato mosaic disease virus is thread-like; the poliomyelitis and turnip yellows mosaic viruses are polyhedral spheres; and the rabies virus and the wheat and lucerne mosaic viruses resemble the rods of bacteria or bullets. By size viruses are divided into large (300-400 nanometers in diameter), medium-sized (80-125 nm), and small (20-30 nm). Large viruses can be seen under a light microscope (standard, phase-contrast, and lumines-cent); the other viruses can be studied only by electron microscopy. Data on the sizes of virus particles have been obtained by the methods of ultrafiltration, preparative and analytical ultracentrifugation, gel electrophoresis, and electron microscopy (see Table 1).

Regarding the structure, different viruses have many common features. They all have a protein coat, the capsid, and a core—the nucleocapsid, consisting primarily of nucleic acid (DNA or RNA). The protein coat of many viruses has an outer envelope. The protein coat of the virus particle is com-posed of subunits called capsomeres. In certain viruses (for example, the tobacco mosaic virus) the nucleic acid forming a helix is so intimately related to the protein coat that it can be isolated only after the destruction of the coat. In other viruses (for example, the turnip yellows mosaic virus) the helices of nucleic acid lie freely in the capsid as if in a box and can be removed without the destruction of the capsular proteins. Nucleic acids are the carriers of genetic information determining the structure and properties of the virus; proteins of the virus protect the nucleic acid and also give the viruses their enzymatic and antigenic properties. The structure of virus particles adapted to survive unfavorable conditions may be still more complex. Examples are the polyhedrons formed by certain insect viruses (they consist of a coat, a crystalline protein substance, and the virus particles included in the protein substance).

The chemical composition of different viruses varies. Some viruses contain lipids. Among them are DNA viruses (such as smallpox and herpes viruses) as well as RNA viruses (influenza, poultry plague, Rous sarcoma, tomato bronzing, and potato yellow dwarf viruses). Other viruses do not contain any lipids. This group also includes DNA viruses (adenoviruses, the majority of bacteriophages, silkworm jaundice virus) and RNA viruses (poliomyelitis and foot-and-mouth disease viruses, the majority of viruses that cause plant diseases, and certain bacteriophages). In addition to lipids, protein, and nucleic acid, certain viruses contain small quantities of polyamines (such as putrescine and spermidine) or vitamins (vitamin B2, folic acid) and a number of metals. Some viruses contain glycoproteins.

Reproduction.Viruses multiply inside the cells. Bacteriophages penetrate the cell membrane of the bacterium and introduce the strands of nucleic acid into the bacterium, with the capsid of the phage remaining outside the cell. Many viruses are absorbed by the cell by means of pinocytosis. Having entered the cell, they strip off the coat. The first stages of virus development in the cell, in general outline, involve the biosynthesis of so-called early proteins, that is, the enzymes necessary for replication (duplication) of the nucleic acids of the virus. The so-called late proteins are involved in the synthesis of the protein coats of the virions. One of the first enzymes synthesized in viruses that contain DNA is RNA polymerase, which catalyzes synthesis of messenger RNA on the DNA template. This RNA attaches to the ribosomes of the (host) cell, on which the synthesis of other proteins of the virus particle takes place. Viruses composed of RNA synthesize a polymerase, which catalyzes the synthesis of new RNA strands. RNA then is attached to the ribosomes and controls the synthesis of capsid proteins. So RNA viruses do not need DNA for reproduction and for transfer of genetic information to their progeny (see Figure 1).

Figure 1. Diagram of the reproduction of viruses that contain a single-stranded DNA (I) or RNA (II) in the virion. The DNA is represented by a solid line; the RNA is represented by a dotted line, (a) Nucleic acid of the virion. (b) Double-stranded nucleic acid after replication, (c) Messenger RNA that copies the viral DNA. (d) Chain of ribosomes (polysome) that is connected by the messenger RNA or viral RNA (a polypeptide chain is synthesized on the ribosomes from the amino acid residues), (e) Ribosome with a polypeptide, which has been detached from the polysome. (f) Protein molecule composed of poly-peptide chains, (g) Transcription of the single-stranded nucleic acid on two strands of a template nucleic acid, (h) Mature virion. In RNA viruses stage (c) is absent because their own RNA plays the role of messenger RNA during protein synthesis.

There are some deviations from this general scheme of viral reproduction. For example, some viruses possess enzymes, and the vaccinia virus synthesizes double-stranded RNA in the host cell. Many steps of viral reproduction have not yet been clarified. For example, there are special sites in the cell where replication of viral nucleic acids takes place. During maturation of the virus particles proteins are synthesized that encapsulate individual segments of the nucleic acid. Sometimes this process is deficient, and incomplete virus particles are formed without any or with only a small amount of nucleic acid and these are so-called noninfectious viruses. In many cases the sites of viral reproduction in the cell can be easily seen under a microscope. These sites are called intracellular inclusions or X-bodies. During maturation, virions occur in the X-body. The virions of many viruses form crystalline aggregates in the X-bodies, but in other viruses this does not happen. Some viruses multiply in the nucleus; others, in the cytoplasm of the cell; while still others multiply in both the nucleus and the cytoplasm. Nucleic acid is found in the virospore in the form of a helix. The length of the nucleic acid strand varies in different viruses. For example, in the smallpox virus it reaches 83 micrometers (μm) and for large bacteriophages such as T₄, 70 μm. In the smallest bacteriophages the nucleic acid thread is about 2 μm long. Depending on the length of the nucleic acid strand (which determines the amount of genetic information that the particular virus has), that is, depending on the capability of the virus to synthesize various large or small protein molecules, the components of the host cell participate to a different degree in virus growth and reproduction. Viruses that have nucleic acid strands of significant length can synthesize many substances. For example, certain bacteriophages synthesize several dozen different proteins in the cell. All DNA viruses synthesize the corresponding RNA. Even if the host cell has the necessary enzymes, the virus frequently synthesizes its own enzymes with activities similar to those of the cell. The smallest phages have information for synthesizing only three proteins of their own; for example, the phage MZ-2 synthesizes a RNA-dependent polymerase and two proteins necessary for building mature viral particles. Thus the degree of dependence of viruses on different enzymes of the host cell varies. Some viruses are so poor in hereditary (genetic) information that they can reproduce in the cell only in the presence of other viruses. This dependence of a virus on both the cell and on another virus exists, for example, between the tobacco necrosis virus and its associated virus, whose virions are smaller than the tobacco necrosis virions. Even closer mutual relations exist between certain viruses that infect humans and animals. Among the viruses capable of causing malignant tumors, there are viruses with a defective particle that cannot form its own protein coat. These viruses reach a mature state only if they reproduce in the presence of other viruses (such are the relations, for example, between the simian tumorigenic virus S-40 and certain adenoviruses). In this case the nucleic acid of the tumorigenic virus is incorporated in the capsid of the adenovirus and enters the susceptible cell together with it. In some cases, the escape of the virus from the cell is always accompanied by the lysis of the cell (many phages, the smallpox virus), but in other cases the virus particles leave the cell without killing it (myxoviruses, certain small phages).

If viruses that differ by particular genes (the differences may be the result of mutation) enter the cell, it is possible to observe in the progeny viruses that combine the properties of two and more initial forms. This indicates that an exchange (recombination) of the genetic information of such forms occurs during viral reproduction in one cell. Viral genetics studies the laws of these processes.

The resistance of virospores to external influence varies, but for the most part it is high. Certain viruses are inactivated only when heated to 90° C (the tobacco mosaic disease virus) and easily endure very low temperatures (—70° C and lower) and dessication.

Infection. The ways in which viruses are transmitted in nature vary. Many of them can infect a susceptible organism directly (the flu, smallpox, and tobacco mosaic disease viruses and bacteriophages); the circulation of others in nature is more complex, and they are transmitted by means of other vectors. For example, the tobacco necrosis virus is transmitted by means of a small fungus (Olpidium) that grows in the soil. Penetrating the roots of a plant, the fungus carries the virus in. Many viruses are transmitted by nematodes that live as parasites on plants. Ticks and insects also transmit animal, human, and plant viruses. Some viruses are transmitted mechanically by biting arthropods; in other cases part of the development of the virus takes place in the vector, and the virus may even be passed along from generation to generation with the eggs of the carrier. Many viruses that infect humans and domestic animals also live in wild animals, and viruses that infect cultivated plants live in wild plants and weeds.

Attempts to find signs of life of the virions outside the cell have, naturally, not been successful; it is well known that resting forms of living organisms in general do not manifest life. It is possible to reproduce certain phases of viral growth, in an acellular system, for example, to obtain self-duplication of viral nucleic acid and to synthesize the proteins typical for the virus under the control of this particular nucleic acid. But these processes occur only in the presence of ribosomes isolated from a cell. Therefore, although these systems are acellular, they cannot be considered completely artificial.

The origin of viruses. There are various theories concerning the origin of viruses. Some feel that viruses can be spontaneously generated in the host organism under the influence of unfavorable conditions. But this opinion is refuted by the evidence of the long evolution of viruses (their adaptation to circulation in nature) and also by the absence of transitional forms between viruses and cell organelles. Other investigators think that viruses are residues of the simplest forms of life, but this theory, too, is unlikely because the evident parasitic nature of viruses presumes the existence of more highly organized beings in which the viruses could live and reproduce. Therefore the most probable theory is that viruses originated from freely living, more complex forms and that the simplicity of viruses is secondary, a result of adaptation to a parasitic way of life. Such simplicity secondary to loss of ability for self-nourishment and an intensified capability for reproduction is in general very typical of parasites. Another argument in favor of the long existence of viruses and their gradual evolution is the fact that they take part in complex interrelationships with other types of animals and plants (transmissive viruses that are transmitted by different organisms).

Classification of viruses. There is no generally accepted classification and nomenclature for viruses. They are given generic and specific names as are other animals and plants; popular names and various abbreviations are used or the generic name of the organism infected by the virus and a number are employed (for example, Nicotiana virus 1 is the tobacco mosaic disease virus). Therefore, each virus may have several names. The first attempt to classify viruses was made by the Austrian zoologist S. Prowazek (1907). He included viruses among the living organisms in the Chamydozoa group. By the middle of the 20th century three main classification systems had been formulated. The defenders of one system make the properties of the virions the basis of the classification; they take into consideration the presence of RNA or DNA in the virions, the symmetry of the nucleocapsid, the presence or absence of a peplos (special capsid coat), the diameter of the nucleocapsid (in spiral virions), and the number of facets and capsomeres (in cubical virions). Representatives of the second system (the numeric system) consider as many attributes as possible and group together the viruses showing maximum similarities. Followers of the third system preserve the principles of classical systematization and place viruses in groups on the basis of fundamental attributes that suggest their common origin (chemical similarity, similarities in morphological stages of development, and ways of recycling in nature). The International Virus Nomenclature Committee proposes the use of a binary nomenclature, that is, adding the word “virus” to the generic name (for example, the genus of the smallpox virus is Poxvirus). Many names are commonly used even though they do not correspond to the binary nomenclature. Supporters of the numeric system propose the use of cryptograms that describe the most important properties of viruses by arbitrary symbols. For example, the tobacco mosaic virus is written as The first symbol shows that this virus contains RNA (R) and that the RNA consists of a single-stranded molecule (1); the second symbol gives the molecular weight of the RNA in millions and the percentage of RNA in the virus particle; the third indicates that the particle has an elongated shape of even width, ends that are not rounded, and also that the nucleocapsid has a similar shape; the last symbol indicates that the virus infects higher plants (S) and is spread without a carrier (0).