A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat. Viruses are parasites; they depend on cells for molecular building blocks, machinery and energy. Virus particles are small; dimensions range from approx. 20–400 nm. Viruses infect all cellular life forms: eukaryotes (vertebrate animals, invertebrate animals, plants, fungi) and prokaryotes (bacteria and archaea). The viruses that infect prokaryotes are often referred to as bacteriophages,or phages for short. Virus was first discovered by Dmitri Ivanowsky in 1892. He recognised an infectious agent, which caused tobacco mosaic disease and were smaller than bacteria. M.W. Beijerinek in 1898 called the filter 'Contagium vivum fluidum' and named it the 'virus'.

A virion contains the genome of a virus in the form of one or more molecules of nucleic acid. For any one virus the genome is composed of either RNA or DNA. Each nucleic acid molecule is either single-stranded (ss) or double-stranded (ds), giving four categories of virus genome: dsDNA, ssDNA, dsRNA and ssRNA. The dsDNA viruses encode their genes in the same kind of molecule as animals, plants, bacteria and other cellular organisms, while the other three types of genome are unique to viruses. It interesting to note that most fungal viruses have dsRNA genomes, most plant viruses have ssRNA genomes and most prokaryotic viruses have dsDNA genomes. The reasons for these distributions presumably concern diverse origins of the viruses in these very different host types. Genome sizes of bacteriophages and viruses range from about 2kb to over 1Mb. Virus genomes span a large range of sizes. Porcinecircovirus (ssDNA) and hepatitis delta virus (ssRNA) each have a genome of about 1.7 kilobases (kb), while at the other end of the scale there are viruses with dsDNA genomes comprised of over 1000 kilobase pairs (kbp).

Proteins that are components of virions are known as structural proteins. They have to carry out a wide range of functions, including:

Virus proteins may have additional roles, some of which may be carried out by structural proteins, and some by non-structural proteins (proteins synthesized by the virus in an infected cell but they are not virion components). These additional roles include:

Outside their host cells, viruses survive as virus particles, also known as virions. The virion is a gene delivery system; it contains the virus genome, and its functions are to protect the genome and to aid its entry into a host cell, where it can be replicated and packaged into new virions. In a virion the virus genome is enclosed in a protein coat, known as a capsid. For some viruses the genome and the capsid constitute the virion, while for other viruses there are additional components. There may be an envelope at the surface of the virion, in which case there may be protein between the envelope and the capsid, or there may be an internal lipid membrane. A few viruses produce protein occlusion bodies in which virions become embedded.

Virus genomes removed from their capsids are more susceptible to inactivation, so a major function of the capsid is undoubtedly the protection of the genome. A second major function of many capsids is to recognize and attach to a host cell in which the virus can be replicated. Although the capsid must be stable enough to survive in the extracellular environment, it must also have the ability to alter its conformation so that, at the appropriate time, it can release its genome into the host cell. For many viruses the capsid and the genome that it encloses constitute the virion. For other viruses a lipid envelope, and sometimes another layer of protein, surrounds this structure, which is referred to as a nucleocapsid. Capsids are constructed from many molecules of one or a few species of protein. The individual protein molecules are asymmetrical, but they are organized to form symmetrical structures. A symmetrical object, including a capsid, has the same appearance when it is rotated through one or more angles, or when it is seen as a mirror image. For the vast majority of viruses the capsid symmetry is either helical or icosahedral. The capsids of some viruses, such as papillomaviruses, are clearly constructed from discrete structures. These structures are called capsomeres and each is built from several identical protein molecules.

The capsids of many ssRNA viruses have helical symmetry; the RNA is coiled in the form of a helix and many copies of the same protein species are arranged around the coil. This forms an elongated structure, which may be a rigid rod if strong bonds are present between the protein molecules in successive turns of the helix, or a flexible rod if these bonds are weak. The length of the capsid is determined by the length of the nucleic acid. For many ssRNA viruses, such as measles and influenza viruses, the helical nucleic acid coated with protein forms a nucleocapsid, which is inside an envelope. The nucleocapsid may be coiled or folded to form a compact structure. The virions of some plant viruses that have helical symmetry (e.g. tobacco mosaic virus) are hollow tubes; this allows the entry of negative stain, making the centre of the virion appear dark in electron micrographs. The rod-shaped tobacco rattle virus has a segmented genome with two RNAs of different sizes packaged in separate virions, resulting in two lengths of virion. The virions of a few DNA viruses, such as the filamentous phages, also have helical symmetry.The capsids of many ssRNA viruses have helical symmetry; the RNA is coiled in the form of a helix and many copies of the same protein species are arranged around the coil. This forms an elongated structure, which may be a rigid rod if strong bonds are present between the protein molecules in successive turns of the helix, or a flexible rod if these bonds are weak. The length of the capsid is determined by the length of the nucleic acid. For many ssRNA viruses, such as measles and influenza viruses, the helical nucleic acid coated with protein forms a nucleocapsid, which is inside an envelope. The nucleocapsid may be coiled or folded to form a compact structure. The virions of some plant viruses that have helical symmetry (e.g. tobacco mosaic virus) are hollow tubes; this allows the entry of negative stain, making the centre of the virion appear dark in electron micrographs. The rod-shaped tobacco rattle virus has a segmented genome with two RNAs of different sizes packaged in separate virions, resulting in two lengths of virion. The virions of a few DNA viruses, such as the filamentous phages, also have helical symmetry.

Before proceeding further, a definition of the term ‘icosahedron’ is required. An icosahedron is an object with

An icosahedron has five-, three- and two-fold axes of rotational symmetry. Capsids with icosahedral symmetry consist of a shell built from protein molecules that appear to have been arranged on scaffolding in the form of an icosahedron. They have less contact with the virus genome than the capsid proteins of viruses with helical symmetry.

Many viruses have a lipid membrane component. In most of these viruses the membrane is at the virion surface and is associated with one or more species of virus protein. This lipid–protein structure is known as an envelope and it encloses the nucleocapsid (nucleic acid plus capsid). The virions of most enveloped viruses, such as herpesviruses, are spherical or roughly spherical. Some viruses have a membrane located not at the virion surface, but within the capsid.