Virus

A virus is a small, infectious agent that consists of a core of genetic material (either deoxyribonucleic acid [DNA] or ribonucleic acid [RNA]) surrounded by a shell of protein. Viruses cause disease by infecting a host cell and commandeering the host cell's synthetic capabilities to produce more viruses. The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the host. Because viruses invade cells, no drug therapy has yet been designed to kill viruses. The human immune system is the only defense against a viral disease.

Viruses can infect both plants, bacteria, and animals. The tobacco mosaic virus, one of the most studied of all viruses, infects tobacco plants. Bacterial viruses, called bacteriophages, infect a variety of bacteria, such as Escherichia coli, a bacteria commonly found in the human digestive tract. Animal viruses cause a variety of fatal diseases. Acquired Immune Deficiency Syndrome (AIDS) is caused by the Human Immunodeficiency Virus (HIV); hepatitis and rabies are viral diseases; and the socalled hemorrhagic fevers, which are characterized by severe internal bleeding, are caused by filoviruses. Other animal viruses cause some of the most common human diseases. Often, these diseases strike in childhood. Measles, mumps, and chickenpox are viral diseases. The common cold and influenza are also caused by viruses. Finally, some viruses can cause cancer and tumors. One such virus, Human T-cell Leukemia Virus (HTLV), was only recently discovered and its role in the development of a special kind of leukemia is still being elucidated.

Although viral structure varies considerably between the different types of viruses, all viruses share some common characteristics. All viruses contain either RNA or DNA surrounded by a protective protein shell called a capsid. Some viruses have a double strand of DNA, others a single strand of DNA. Other viruses have a double strand of RNA or a single strand of RNA. The size of the genetic material of viruses is often quite small. Compared to the 100,000 genes that exist within human DNA, viral genes number from 10 to about 200 genes.

Viruses contain such small amounts of genetic material because the only activity that they perform independently of a host cell is the synthesis of the protein capsid. In order to reproduce, a virus must infect a host cell and take over the host cell's synthetic machinery. This aspect of viruses—that the virus does not appear to be "alive" until it infects a host cell—has led to controversy in describing the nature of viruses. Are they living or non-living? When viruses are not inside a host cell, they do not appear to carry out many of the functions ascribed to living things, such as reproduction, metabolism, and movement. When they infect a host cell, they acquire these capabilities. Thus, viruses are both living and nonliving. It was once acceptable to describe viruses as agents that exist on the boundary between living and non-living; however, a more accurate description of viruses is that they are either active or inactive, a description that leaves the question of life behind altogether.

The origin of viruses is also controversial. Some viruses, such as the pox viruses, are so complex that they appear to have been derived from some kind of living eukaryote or prokaryote. The origin of the poxvirus could therefore resemble that of mitochondria and chloroplasts, organelles within eukaryotic cells which are thought to have once been independent organisms. On the other hand, some viruses are extremely simple in structure, leading to the conclusion that these viruses are derived from cellular genetic material that somehow acquired the capacity to exist independently. This possibility is much more likely for most viruses; however, scientists still believe that the poxvirus is the exception to this scenario.

Scientists have classified viruses according to the type of genetic material they contain. Broad categories of viruses include double-stranded DNA viruses, single-stranded DNA viruses, double-stranded RNA viruses, and single stranded RNA viruses. For the description of virus types that follows, however, these categories are not used. Rather, viruses are described by the type of disease they cause.

These helical, enveloped, single-stranded RNA viruses cause pneumonia, croup, measles, and mumps in children. A vaccine against measles and mumps has greatly reduced the incidence of these diseases in the United States. In addition, a paramyxovirus called respiratory syncytial virus (RSV) causes bronchiolitis (an infection of the bronchioles) and pneumonia.

Flaviviruses (from the Latin word meaning "yellow") cause insect-carried diseases including yellow fever, an often fatal disease characterized by high fever and internal bleeding. Flaviviruses are single-stranded RNA viruses.

The two filoviruses, Ebola virus and Marburg virus, are perhaps the most lethal of all human viruses. Both cause severe fevers accompanied by internal bleeding, which eventually kills the victim. The fatality rate of Marburg is about 60%, while the fatality rate of Ebolavirus is about 90%. Both are transmitted through contact with body fluids. Marburg and Ebola also infect primates.

Rhabdoviruses are bullet-shaped, single-stranded RNA viruses. They are responsible for rabies, a disease that affects dogs, rodents, and humans.