Virus is neither a plant nor an animal or a prokaryotic bacteria and it has a special taxonomic position. In fact viruses should not even be considered as organisms because they re not free living.
All viruses contain nucleic acid, either DNA or RNA and protein. The nucleic acid encodes the genetic information unique for each virus. The infective, extra cellular form of a virus is called virion. It contains at least one unique proteins synthesized by specific genes in the nucleic acid of that virus. In virtually all viruses, at least one of these proteins forms a shell around the nucleic acid. Certain viruses also have other proteins internal to the capsid. Some of these proteins act as enzymes, often during the synthesis of viral nucleic acids. Viroids are disease-causing organisms that contain only nucleic acid and have no structural proteins.
Viruses are quintessential parasites. They depend on the host cell for almost all their life sustaining functions. Unlike true organisms, viruses cannot synthesize proteins because they lack ribosome for the translation of messenger RNA into proteins. Viruses must use the ribosome of their host cells to translate viral MRNA into viral proteins.
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Viruses are also energy parasites. Unlike cells, they cannot generate or store energy in the form of adenosine troposphere. The virus derives energy and all other metabolic functions, from the host cell. The invading virus uses the nucleotides and amino acids of the host cell to synthesize its nucleic acids and proteins, respectively. Some viruses use the lipids and sugar chains of the host cell to form their membranes and glycoprotein.
Virtually all plant viruses are transmitted by insects or other vectors that feed on plants. The hosts of animal viruses very from protozoa to human beings. Many viruses infect either invertebrate animals to vertebrates and some infect both. Certain viruses that cause serious diseases of animals and humans are carried by arthropods. These vector born viruses, which cause human diseases such as yellow fever and encephalitis, multiply both in the invertebrate vector and the vertebrate host. Certain viruses are limited in their host range to the various orders of vertebrates. Some viruses appear to be adapted for growth only in cold-blooded vertebrates, possibly because they cannot reproduce except at low temperatures. Others are limited in their host range to warm-blooded vertebrates.
Viruses can reproduce only within a host cell. The parental virus gives rise to numerous progeny, usually genetically and structurally identical to the parent virus. The action of the virus depend both on its destructive tendencies towards a specific host cell and on environmental conditions. In the vegetative cycle of viral infection, multiplication of progeny viruses can be rapid. This cycle of infection often results in the death of the cell and the release of many virus progeny. Certain viruses, particularly bacteriophages are called temperate because the infection does not immediately result in cell death. The viral genetic material remains dormant or is actually integrated into the genome of the host cell. Cells infected with temperate viruses are called lysogenic because the cells tend to be lysed when they encounter some chemical or physical factor, such as ultraviolet light. In addition, many animal and plant viruses, the genetic information of which is not integrated into the host DNA, may lie dormant in tissues for long periods of time without causing much tissue damage.
Although viruses were originally discovered and characterized because of the diseases they cause, most viruses that infect bacteria, plants and animals do not cause disease. In fact, bacteria, plants and animals do not cause disease. In fact, bacteriophages may be helpful in that they rapidly transfer genetic information from one bacterium to another and viruses of plants and animals may convey genetic information among similar species aiding the survival of their hosts in hostile environments. This could in future be true for humans as well. Recombinant DNA biotechnology may allow genetic defects to be repaired by injecting afflicted persons with harmless viruses that carry and integrate functional genes to supplant defective ones, Such events may actually occur in the transmission of good viruses from one person to another.
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Of those viruses that cause disease, some cause short term diseases and others recurring or long term diseases. Some viruses cause acute disease from which there is fairly rapid recovery, but the same virus may persist in the tissues, remaining dormant for long period and then became active again brining about a series disease decades alter.
The spread of many viral diseases can be prevented by hygienic practices like efficient sanitation facilities, effective waste disposal, clean water and personal cleanliness. Active immunization by vaccines has been useful in preventing common epidemic caused by acutely infectious viruses.
Vaccination can prevent disease caused by strictly human viruses that exist in only one antigenic and stable type. Measles has been prevented in developed countries with vaccination. Vaccination for mumps and chicken pox promises to be successful because the causative viruses of these diseases show little tendency to vary antigenically and are confined to humans. On the other hand, development of vaccines for the common cold caused by rhinoviruses, similar to polioviruses will be a formidable, if not impossible task, because there are at least 100 antigenic types of the rhinovirus.
Unlike bacteria, viruses mimic the metabolic functions of their host cells. Antibiotic and other antimicrobial agents are, therefore, ineffective against viruses because the chemical compounds that inhibit the multiplication of viruses generally also slow the function of, and are toxic to the host cell. so man-made compounds like ribavirin, acyclovir or azidothymidine (AZT) are only partially effective during treatment.