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Viruses: Origin, Classification and Role in the Environment

The word "virus" comes from Latin and means literally "poison." Viruses are usually associated with negative effects. Without viruses, though, the Earth would be a different place, maybe even a planet without life. Viruses are the most abundant form of life on Earth, with a population now estimated at 1030 - a billion billion trillion. Most of the diversity of life's genetic information may reside in their genome. Within the human gut alone there are about a thousand species of viruses.5 Viruses are the most abundant biological entity in both freshwater and seawater. More than 1 million viruses are present in a teaspoon of water. Most of these viruses are known as bacteriophages (viruses that infect bacteria). For each bacterium in the water there 15 to 25 virus particles.

Viruses are the smallest and simplest infectious agents identified to date. They are little more than a piece of genetic material protected by a protein coat. Viruses are about 500 smaller than a bacteria and can only be seen with an electron microscope which can magnify 100,000 times and higher. Many of the small viruses behave like chemicals and can be crystallized.

The Origin of Viruses

Nobody knows for sure where viruses came from. Some scientists speculate that these particles of genetic material broke away from genes in animals and plants and found a way to exist as parasites. The DNA contains "jumping genes," snippets of DNA that can detach from one part of a DNA strand and reattach at another site. Some jumping genes contain instructions for making useful proteins. It is believed that some of these gene segments found a way to leave their "home" cells and to exist parasitically as viruses. In the early stages of life on Earth, these genes may have moved between organisms, increasing genetic diversity and aiding evolution. Now they exist independently, but they need hosts to reproduce.3

Virus Replication

Viruses force their hosting cells to reproduce their genes for them. Genes consist of strands of deoxyribonucleic acid, also known as DNA. Cells are selective about which chemicals they let in and which they keep out. Special receptors on the cell's surface permit only of those chemicals the cell needs. After attaching to the receptor, the virus either enters the cell or injects its genetic material into it. The virus uses the cell's proteins and enzymes to replicate viral genes and build new viral particles. These new viruses leave the host cell and invade other cells.

Classification of Viruses

Historically, viruses were first classified by the diseases they caused, but this led to much confusion because a single virus can be responsible for several diseases. In addition, several unrelated viruses can produce similar diseases. For example, viruses that cause hepatitis (liver swelling and resulting jaundice) were called hepatitis viruses. Yet it is now known that these viruses have very different structure. Classification according to appearance under an electron microscope was more successful. However, this also created confusion because different virus species may look alike. Currently, viruses are classified according to the type of nucleic acid in their genes (double-stranded DNA, single-stranded DNA, double-stranded RNA, and single-stranded RNA), type of replication and the structure of the virus particle. These features are usually maintained during virus evolution.2

The International Committee on Taxonomy of Viruses (ICTV) is attempting to devise a uniform system for the classification and naming of all viruses. Viruses are classified into species on the basis of a close relationship which are then grouped into a genus; genera are grouped into families. The ICTV has recognized more than 5450 viruses as species.4

Viruses Versus Bacteria

Bacteria are the blue print for cells which make up all other animals and plants. They are the smallest microscopic organisms that can survive without help from any other living thing, most of them being just single cells which live in the natural environment. Everyday they are busy breaking down dead plants and animals into their component parts ready for use. The gases released make up our atmosphere, and simple molecules are rebuilt into new plants and animals. Unlike bacteria, viruses can do absolutely nothing on their own. They are not cells but particles and they have no source of energy. They invade living things, commandeer their cells, and turn them into factories for virus reproduction.1

Viruses and Viroids

One of the characteristics that viruses and viroids have in common is that they both produce diseases in their hosts. Viruses and viroids also have several important structural and functional differences.

Viruses are submicroscopic spherical, rod-shaped, or filamentous entities that consist of only one type of nucleic acid (DNA or RNA). The nucleic acid is surrounded by a coat consisting of one or more kinds of protein molecules. Viruses infect and multiply inside the cells of different types of organisms.

Viroids are the smallest infectious agents that multiply autonomously in plant cells; they consist of only small, circular RNA molecules that are too small to code for even one protein and therefore lack a protein coat and are "naked." About 25 viroids have been identified.

Viruses often have methylated ribonucleotides in their genomes, whereas viroid RNA lacks modified bases. All virus RNAs are subject to host cell's enzymatic degradation; viroid genomes are resistant.

Because viroids do not possess any molecules or structures to facilitate their entry into a host, they depend on wounding of a plant, for example, by an insect, to gain entry.[6]

Viruses That Cause Tumors

As of now, viruses have been implicated in the formation of at least eight human cancers:

  1. The Epstein-Barr virus (EBV) is the cause of two cancers, Burkitt's lymphoma (a malignant tumor of the jaw and abdomen in children of central and western Africa) and nasopharyngeal carcinoma.
  2. Hepatitis B virus causes hepatocellular carcinoma (liver cancer).
  3. Hepatitis C virus causes cirrhosis of the liver which can lead to liver cancer.
  4. Human herpes virus 8 is associated with Kaposi sarcoma.
  5. Some strains of human papillomaviruses have been linked to cervical cancer.
  6. Human T-cell lymphotropic virus I (HTLV-1) causes adult T-cell leukemia.
  7. Human T-cell lymphotropic virus II (HTLV-2) causes hairy-cell leukemia.

With the possible exception of HTLV-1, little is known how the viruses associated with human cancers actually aid in cancer development.[7]

Viruses Infecting Man And Other Vertebrates.

  • DNA Viruses
    • Human Adenoviruses - cause a wide range of diseases in humans. Infections are mostly asymptomatic, but can be associated with diseases of the respiratory, ocular, and gastrointestinal systems.
    • Anellovirus - free-floating DNA virus genus, unattached to any family, comprising several species of hepatitis-related viruses.
    • Hepatitis B virus - causes human hepatitis B and is also apparently a causal agent in human hepatocellular carcinoma.
    • Human Herpesvirus 1 - species of Simplexvirus causing most forms of non-genital herpes simplex in humans. Primary infection occurs mainly in infants and young children and then the virus becomes latent in the dorsal root ganglion. It then is periodically reactivated throughout life causing mostly benign conditions.
    • Human Herpesvirus 2 - associated with genital infections (HERPES GENITALIS). It is transmitted by sexual intercourse and close personal contact.
    • Human Herpesvirus 3 - species of Varicellovirus causing chickenpox (varicella) and Herpes zoster (shingles) in humans.
    • Human Herpesvirus 4 infects B-cells in humans. It is thought to be the causative agent of infectious mononucleosis and is strongly associated with oral hairy leukoplakia (hairy leukoplakia), Burkitt lymphoma and other malignancies.
    • Human Herpesvirus 6 infects and replicates in fresh and established lines of hematopoietic cells and cells of neural origin. It also appears to alter Natural Killer cell activity. HHV-6; (HBLV) antibodies are elevated in patients with AIDS, Sjogren's syndrome, sarcoidosis, chronic fatigue syndrome, and certain malignancies. HHV-6 is the cause of exanthema subitum and has been implicated in encephalitis.
    • Human Herpesvirus 7 was isolated from activated, CD4-positive T-lymphocytes taken from the blood of a healthy human.
    • Human Herpesvirus 8 was isolated from patients with AIDS-related and "classical" Kaposi sarcoma.
    • Cytomegalovirus - infects the salivary glands, liver, spleen, lungs, eyes, and other organs, in which they produce characteristically enlarged cells with intranuclear inclusions. Infection with Cytomegalovirus is also seen as an opportunistic infection in AIDS.
    • Human papillomavirus 16 - especially associated with malignant tumors of the cervix and the respiratory tract.
    • Human papillomavirus 18 - especially associated with malignant tumors of the cervix and the respiratory tract.
    • Human papillomavirus 6 - especially associated with genital warts and laryngeal cancers.
    • Human papillomavirus 11- causs recurrent respiratory papillomatosis, genital warts and other neoplasms.
    • Betapapillomavirus - causes skin lesions in humans. Infections exist in latent form in the general population and are activated under conditions of immunosuppression.
    • Gammapapillomavirus - causes skin lesions in humans.
    • Lambdapapillomavirus - causes mouth and skin lesions in cats and dogs. Canine oral papillomavirus is the type species.
    • Mupapapillomavirus - causes skin lesions in humans.
    • Human bocavirus - causes respiratory disease.
    • Human Parvovirus B19 the etiological agent of erythema infectiosum, a disease most commonly seen in school-age children.
    • Feline panleukopenia virus - infects cats with a highly contagious enteric disease. Host range variants include mink enteritis virus, canine parvovirus, and raccoon parvovirus. After infecting their new hosts, many of these viruses have further evolved and are now considered distinct species.
    • Canine parvovirus - causes a highly infectious fulminating enteritis in dogs producing high mortality. It is distinct from canine minute virus, a species in the genus bocavirus. This virus can also infect cats and mink.
    • BK Virus - a species of Polyomavirus infecting over 90% of children but not clearly associated with any clinical illness in childhood. The virus remains latent in the body throughout life and can be reactivated under certain circumstances.
    • JC Virus - a species of Polyomavirus, originally isolated from the brain of a patient with progressive multifocal leukoencephalopathy. Infection is not accompanied by any apparent illness but serious demyelinating disease can appear later, probably following reactivation of latent virus.
    • Merkel cell polyomavirus - a species of Polyomavirus suspected to be the cause of most cases of merkel cell carcinoma, a rare but highly lethal form of skin cancer.
    • Simian virus 40 - a species of Polyomavirus originally isolated from Rhesus monkey kidney tissue. It produces malignancy in human and newborn hamster kidney cell cultures.
  • RNA Viruses
    • Human astrovirus - causes gastroenteritis.
    • Sin Nombre virus - a species of Hantavirus; causes a serious, often fatal pulmonary illness (hantavirus pulmonary syndrome) in humans. Transmission is by inhaling aerosolized rodent secretions that contain virus particles, carried especially by deer mice and pinyon mice.
    • Nairovirus - causes an acute, hemorrhagic, tick-borne, gastroenteritis affecting sheep and goats. The type species is Dugbe virus. Some viruses in this genus are capable of causing severe and fatal disease in humans.
      • Crimean-Congo Hemorrhagic Fever Virus - is primarily transmitted by ticks and causes a severe, often fatal disease in humans.
      • Nairobi sheep disease virus - a species of nairovirus, transmitted by the ixodid ticks and producing a lethal gastroenteritis in sheep and goats. Though a major veterinary pathogen, its effect on humans has not been firmly established.
    • Bunyamwera virus - are transmitted by mosquitoes and infect humans in some areas.
    • California Encephalitis Virus - infects small mammals but several serotypes infect humans. Its subspecies La Crosse virus causes human meningoencephalitis. This is the agent most responsible for California encephalitis, the most prevalent mosquito-borne disease recognized in the United States.
    • Rift Valley fever virus - a mosquito-borne species of the phlebovirus genus found in eastern, central, and southern Africa, producing massive hepatitis, abortion, and death in sheep, goats, cattle, and other animals. It also has caused disease in humans.
    • Sandfly fever Naples virus species of the phlebovirus genus causing phlebotomus fever, an influenza-like illness. Related serotypes include Toscana virus and Tehran virus.
    • Uukuniemi - a species of the phlebovirus genus infecting vertebrates and vectored by ticks. It has not been associated with human disease though antibodies have been isolated from human sera.
    • Lagovirus - associated with infections in rabbits and hares, responsible for epidemics with high mortality.
    • Norovirus - associated with epidemic gastroenteritis in humans. The type species is norwalk species.


  1. The Invisible Enemy: A Natural History of Virises. Dorothy Crawford
  2. Viruses and the evolution of life. Luis P. Villarreal
  3. Viruses. Natalie Goldstein
  4. Viruses and human disease. James H. Strauss, Ellen G. Strauss
  5. Microcosm by Carl Zimmer<
  6. Plant pathology By George N. Agrios; Symbiosis: an introduction to biological associations By 'Surindar Paracer, Vernon Ahmadjian
  7. Microbiology by Prescott Harley Klein