A virus is a submicroscopic parasitic particle that infects cells in biological organisms. The study of viruses is virology.

Viruses are obligate intracellular parasites that lack the cellular machinery for self-reproduction. Viruses infect eukaryotes and prokaryotes such as bacteria; viruses infecting prokaryotes are also known as bacteriophages or phages. Typically viruses carry a small amount of genetic material, either in the form of DNA or RNA or both, surrounded by some form of protective coat consisting of proteins, lipids, glycoproteins or a combination. The viral genome codes for the proteins that constitute this protective coat, as well as for those proteins required for viral reproduction that are not provided by the host cell.

Viruses are non-living particles that can only reproduce when an organism reproduces the viral RNA or DNA. Viruses are considered non-living by the majority of virologists because they do not meet all the criteria of the generally accepted definition of life. Among other factors, viruses do not move or metabolize on their own. However, a comprehensive definition of life is still somewhat elusive since some bacteria (considered living) like rickettsia exhibit both characteristics of living and non-living particles.

Size, structure, and anatomy:

A virus particle, known as a virion, is little more than a gene transporter, consisting at the most basic level of a genome contained within a protective casing of protein. The nucleic acid genome varies among different viruses and may be either DNA, or RNA; single- or double-stranded; linear or circular; and positive or negative sense. The genetic material is surrounded and thus encapsidated by a protective coat of protein called a capsid. This capsid is composed of proteins encoded by the viral genome and may be either spherical or helical. These proteins are associated with the nucleic acid and are hence known as nucleoproteins, the combined partnership of nucleoproteins and nucleic acid producing what is known as a nucleocapsid.

In addition to a capsid some viruses are able to hijack a modified form of the plasma membrane surrounding an infected host cell, thus gaining an outer lipid bilayer known as a viral envelope. This extra membrane is studded with proteins synthesized by the host cell, which the virus may have modified at a genetic level. This gives the virion a few distinct advantages over “naked” virions - the plasma membrane provides a degree of protection for the virus, especially from harmful agents such as enzymes and chemicals. The proteins studded upon it include glycoproteins, which serve as receptor molecules, allowing healthy cells to recognise virions as “friendly” and resulting in the possible uptake of the virion into the cell.

Helical capsids are composed of identical proteins stacked at a constant amplitude and pitch to one another around a central circumference, much like a spiral staircase, which effectively forms an enclosed tube housing the genetic material. This arrangement results in rod-shaped virions which can either be short and rigid or long and flexible; the nature of long helical particles neccessitates flexibility, as they are prone to damage if they are too rigid.

Spherical virus capsids completely enclose the viral genome and do not generally bind as tightly to nucleic acid as helical capsid proteins do. These structures can range in size from less than 20 nanometers up to 400 nanometers and are composed of viral proteins arranged with icosahedral symmetry, hence are not truly “spherical”. Icosahedral architecture is the same principle employed by R. Buckminster-Fuller in his geodesic dome, and it is the most efficient way of creating an enclosed robust structure from multiple copies of a single protein. The number of proteins required to form a spherical virus capsid is denoted by the “T-number”, where 60×t proteins are necessary. In the case of the Hepatitis B virus, the T-number is 4, therefore 240 proteins assemble to form the capsid. Many spherical viruses forgo a lipid envelope, leaving the capsid proteins to be directly involved in attachment and entry into the host cell.

Study and applications:

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 cells. The study and use of viruses have provided valuable information about many aspects of cell biology. For example, viruses have further simplified the study of genetics and have helped human understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.

Viro-therapy uses viruses as treatment against various diseases, most commonly as a vector. It is not a new idea. In treatments in oncology for example, it was recognized as early as the mid 20th Century, when a number of physicians noticed an interesting phenomenon: some of their patients, who suffered from cancer and had an incidental viral infection, or subjected to vaccination, improved, experiencing a remission from their symptoms. In the 1940s and 1950s, studies were conducted in animal models to evaluate the use of viruses in the treatment of tumors, and in 1956, one of the first human clinical trials with oncolytic viruses was conducted in patients with advanced-stage cervical cancer. Nevertheless, systematic research of this field was delayed for years, due to lack of more advanced technologies. In recent years the research in the field of oncolytic viruses began to move forward more quickly and researchers are trying new ways to use viruses for the therapeutic benefit of patients.

In 2006 researchers from the Hebrew University succeeded in isolating a variant of the Newcastle disease Virus (NDV-HUJ), which usually affects birds, in order to specifically target cancer cells [1]. The researchers tested the new Viro-therapy on Glioblastoma multiforme patients and achieved promising results for the first time.

Genetic engineering:

Geneticists regularly use viruses as vectors to introduce genes into cells that they are studying. Attempts to treat human diseases through the use of viruses as tools of genetic engineering is one goal of gene therapy.
Materials science and nanotechnology

Scientists at the Massachusetts Institute of Technology (MIT) have recently been able to use viruses to create metallic wires, and they have the potential to be used for binding to exotic materials, self-assembly, liquid crystals, solar cells, batteries, fuel cells, and other electronics.

The essential idea is to use a virus with a known protein on its surface. The location of the code for this protein is in a known location in the DNA, and by randomizing that sequence it can create a phage library of millions of different viruses, each with a different protein expressed on its surface. By using natural selection, one can then find a particular strain of this virus which has a binding affinity for a given material.

For example, one can isolate a virus which has a high affinity for gold. Taking this virus and growing gold nanoparticles around it results in the gold nanoparticles being incorporated into the virus coat, resulting in a gold wire of precise length and shape with biological origins.

Current thinking is that viruses will one day be created which can act as agents on behalf of bio-mechanical healing devices giving humans or other animals, notably pets, extended life.