A virus is a microscopic particle (ranging in size from 20 - 300 nm) that can infect the cells of a biological organism.
Viruses can replicate themselves only by infecting a host cell. They therefore cannot reproduce on their own. At the most basic level, viruses consist of genetic material contained within a protective protein coat called a capsid.
They infect a wide variety of organisms: both eukaryotes (animals, yeasts, fungi, and plants) and prokaryotes (bacteria and archaea). A virus that infects bacteria is known as a bacteriophage, often shortened to phage.
The word virus comes from the Latin, poison (syn. venenum).[1] The study of viruses is known as virology. Those who study viruses are known as virologists.
It has been argued extensively whether viruses are living organisms. Most virologists consider them non-living, as they do not meet all the criteria of the generally accepted definition of life. They are similar to obligate intracellular parasites as they lack the means for self-reproduction outside a host cell, but unlike parasites, viruses are generally not considered to be true living organisms. A definitive answer is still elusive. Some organisms considered to be living exhibit characteristics of both living and non-living particles, as viruses do. For those who consider viruses living, viruses are an exception to the cell theory proposed by Theodor Schwann, as viruses are not made up of cells.
Detection, purification and diagnosis
In the laboratory, several techniques for growing and detecting viruses exist. Purification of viral particles can be achieved using differential centrifugation, isopycnic centrifugation, precipitation with ammonium sulfate or ethylene glycol, and removal of cell components from a homogenised cell mixture using organic solvents or enzymes to leave the virus particles in solution.
Assays to detect and quantify viruses include:
Viruses can replicate themselves only by infecting a host cell. They therefore cannot reproduce on their own. At the most basic level, viruses consist of genetic material contained within a protective protein coat called a capsid.
They infect a wide variety of organisms: both eukaryotes (animals, yeasts, fungi, and plants) and prokaryotes (bacteria and archaea). A virus that infects bacteria is known as a bacteriophage, often shortened to phage.
The word virus comes from the Latin, poison (syn. venenum).[1] The study of viruses is known as virology. Those who study viruses are known as virologists.
It has been argued extensively whether viruses are living organisms. Most virologists consider them non-living, as they do not meet all the criteria of the generally accepted definition of life. They are similar to obligate intracellular parasites as they lack the means for self-reproduction outside a host cell, but unlike parasites, viruses are generally not considered to be true living organisms. A definitive answer is still elusive. Some organisms considered to be living exhibit characteristics of both living and non-living particles, as viruses do. For those who consider viruses living, viruses are an exception to the cell theory proposed by Theodor Schwann, as viruses are not made up of cells.
Detection, purification and diagnosis
In the laboratory, several techniques for growing and detecting viruses exist. Purification of viral particles can be achieved using differential centrifugation, isopycnic centrifugation, precipitation with ammonium sulfate or ethylene glycol, and removal of cell components from a homogenised cell mixture using organic solvents or enzymes to leave the virus particles in solution.
Assays to detect and quantify viruses include:
A viral plaque assay
- Hemagglutination assays, which quantitatively measure how many virus particles are in a solution of red blood cells by the amount of agglutination the viruses cause between them. This occurs as many viruses are able to bind to the surface of one or more red blood cells.
Direct counts using an electron microscope. A dilute mixture of virus particles and beads of known size are sprayed onto a special sheet and examined under high magnification. The virions are counted and the number extrapolated to estimate the number of virions in the undiluted mixture. - Plaque assays involve growing a thin layer of host cells onto a culture dish and adding a dilute mixture of virions onto it. The virions will infect and kill the cells they land on, producing holes in the cell layer known as plaques. The number of plaques can be counted and the number of virions estimated from it.
Detection and subsequent isolation of new viruses from patients is a specialised laboratory subject. Normally it requires the use of large facilities, expensive equipment, and trained specialists such as technicians, molecular biologists, and virologists. Often, this effort is undertaken by state and national governments and shared internationally through organizations like the World Health Organization.
Prevention and treatment
Because viruses use the machinery of a host cell to reproduce and also reside within them, they are difficult to eliminate without killing the host cell. The most effective medical approaches to viral diseases so far are vaccinations to provide resistance to infection, and drugs which treat the symptoms of viral infections. Patients often ask for, and physicians often prescribe, antibiotics. These are useless against viruses, and their misuse against viral infections is one of the causes of antibiotic resistance in bacteria. However, in life-threatening situations the prudent course of action is to begin a course of antibiotic treatment while waiting for test results to determine whether the patient's symptoms are caused by a virus or a bacterial infection.
Potential cure for hitherto incurable diseases
Lately, it has been discovered that viruses have the potential to cure many diseases that are often believed to be incurable, for example aging. [12]. Virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy.
Applications
The polio virus
Life sciences
Viruses are important to the study of molecular and cellular biology as 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 simplified the study of genetics and helped human understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.
Geneticists regularly use viruses as vectors to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. In similar fashion, virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy.
Materials science and nanotechnology
In April 2006 scientists at the Massachusetts Institute of Technology (MIT) created nanoscale metallic wires using a genetically-modified virus[13]. The MIT team was able to use the virus to create a working battery with an energy density up to three times more than current materials. The potential exists for this technology to be used in liquid crystals, solar cells, fuel cells, and other electronics in the future.
The reconstructed 1918 influenza virus
Weapons
The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponized for biological warfare. Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory[14]. The smallpox virus devastated numerous societies throughout history before its eradication. It currently exists in several secure laboratories in the world, and fears that it may be used as a weapon are not totally unfounded. The modern global human population has almost no established resistance to smallpox; if it were to be released, a massive loss of life could be sustained before the virus is brought under control.
For more details on this topic, see Biological warfare.
Because viruses use the machinery of a host cell to reproduce and also reside within them, they are difficult to eliminate without killing the host cell. The most effective medical approaches to viral diseases so far are vaccinations to provide resistance to infection, and drugs which treat the symptoms of viral infections. Patients often ask for, and physicians often prescribe, antibiotics. These are useless against viruses, and their misuse against viral infections is one of the causes of antibiotic resistance in bacteria. However, in life-threatening situations the prudent course of action is to begin a course of antibiotic treatment while waiting for test results to determine whether the patient's symptoms are caused by a virus or a bacterial infection.
Potential cure for hitherto incurable diseases
Lately, it has been discovered that viruses have the potential to cure many diseases that are often believed to be incurable, for example aging. [12]. Virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy.
Applications
The polio virus
Life sciences
Viruses are important to the study of molecular and cellular biology as 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 simplified the study of genetics and helped human understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.
Geneticists regularly use viruses as vectors to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. In similar fashion, virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy.
Materials science and nanotechnology
In April 2006 scientists at the Massachusetts Institute of Technology (MIT) created nanoscale metallic wires using a genetically-modified virus[13]. The MIT team was able to use the virus to create a working battery with an energy density up to three times more than current materials. The potential exists for this technology to be used in liquid crystals, solar cells, fuel cells, and other electronics in the future.
The reconstructed 1918 influenza virus
Weapons
The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponized for biological warfare. Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory[14]. The smallpox virus devastated numerous societies throughout history before its eradication. It currently exists in several secure laboratories in the world, and fears that it may be used as a weapon are not totally unfounded. The modern global human population has almost no established resistance to smallpox; if it were to be released, a massive loss of life could be sustained before the virus is brought under control.
For more details on this topic, see Biological warfare.
