A virus (from the latin virus meaning toxin or poison) is a Sub-microscopi infectious agent that is unable to grow or reproduce outside a host cell. Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. The capsid shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an enveloped. Viruses infect cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected.
It has been argued whether viruses are living organisms. Some consider them non-living as they do not meet the criteria of the definition of life. For example, unlike most organisms, viruses do not have cells. However, viruses have genes and evolve by natural selection . They have been described as organisms at the edge of life. Viral infections in human and animal hosts usually result in an immune response and desease. Often, a virus is completely eliminated by the immune system. Antibiotic have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccine that produce lifelong immunity can prevent virus infections.Viral diseases such as rabies, yellow fever and smallpox have affected humans for centuries. There is hieroglyphical evidence of polio in ancient Egyptian medicine, though the cause of this disease was unknown at the time. In the 10th century, Muhammad ibn Zacariya Razi (Rhazes) wrote the Treatise on Smallpox and Measles, in which he gave the first clear descriptions of smallpox and measles In the 1020s, Avicenna wrote The Canon of Medicine, in which he discovered the contagious nature of infection disease such as tuberculosis and sexuallity transmitted disease, and their distribution through bodily contact or through water and soil ; stated that bodily secreation is contaminated by "foul foreign earthly bodies" before being infected; and introduced the method of quaratine a means of limiting the spread of contagious disease.
When the Black Death bubonic plague reached al-Andalus in the 14th century, IBn Khatima discovered that infectious diseases are caused by microorganism which enter the human body. Another 14th century Andalusian physician, Ibm al- khatib (1313-1374), wrote a treatise called On the Plague, in which he stated how infectious disease can be transmitted through bodily contact and "through garments, vessels and earrings." In 1717, Mary Montagu, the wife of an English ambassador to the Ottoman Empire , observed local women inoculating their children against smallpox. In the late 18th century, Edward Jenner observed and studied Miss Sarah Nelmes, a milkmaid who had previously caught compox and was found to be immune to smallpox , a similar, but devastating virus. Jenner developed the smallpox vaccine based on these findings. After lengthy vaccination campaigns, the World Health Organization (WHO) certified the eradication of smallpox in 1979.
In the late 19th century Charles Chamberland developed a porcelain filter with pores small enough to remove cultured bacteria from their culture medium.Dimitri Ivanouski used this filter to study an infection of tobacco plants, now known as tobacco mosaic virus. He passed crushed leaf extracts of infected tobacco plants through the filter, then used the filtered extracts to infect other plants, thereby proving that the infectious agent was not a bacterium. Similar experiments were performed by several other researchers, with similar results. These experiments showed that viruses are orders of magnitudes smaller than bacteria. The term virus was coined by the Dutch microbiologist Marinus Beijerinck who showed, using methods based on the work of Ivanovski, that tobacco mosaic disease is caused by something smaller than a bacterium. He coined the Latin phrase "contagium vivum fluidum" (which means “soluble living germ”) as first the idea of the virus.
In the early 20th century, Frederick Twort discovered that bacteria could be infected by viruses. Felix d' herelle , working independently, showed that a preparation of viruses caused areas of cellular death on thin cell culture spread on agar. Counting the dead areas allowed him to estimate the original number of viruses in the suspension. The invention of Electron microscopy provided the first look at viruses. In 1935 Wendell Stanley crystallised the tobacco mosaic virus and found it to be mostly protein. A short time later the virus was separated into protein and nucleid acid parts. In 1939, Max Delbruck and E.L. Ellis demonstrated that, in contrast to cellular organisms, bacteriophage reproduce in "one step", rather than exponentially.
A major problem for early virologists was the inability to propagate viruses on sterile culture media, as is done with cellular microorganisms. This limitation required medical virologists to infect living animals with infectious material, which is dangerous. The first breakthrough came in 1931, when Ernest William Goodpasture demonstrated the growth of influenza and several other viruses in fertile chicken eggs. However, some viruses would not grow in chicken eggs, and a more flexible technique was needed for propagation of viruses. The solution came in 1949 when John Franklin Enders, Thomas H. Weller and Frederick Champman Robbin together developed a technique to grow polio virus in cultures of living animal cells. Their methods have since been extended and applied to the growth of viruses and other infectious agents that do not grow on sterile culture media.
It has been argued whether viruses are living organisms. Some consider them non-living as they do not meet the criteria of the definition of life. For example, unlike most organisms, viruses do not have cells. However, viruses have genes and evolve by natural selection . They have been described as organisms at the edge of life. Viral infections in human and animal hosts usually result in an immune response and desease. Often, a virus is completely eliminated by the immune system. Antibiotic have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccine that produce lifelong immunity can prevent virus infections.Viral diseases such as rabies, yellow fever and smallpox have affected humans for centuries. There is hieroglyphical evidence of polio in ancient Egyptian medicine, though the cause of this disease was unknown at the time. In the 10th century, Muhammad ibn Zacariya Razi (Rhazes) wrote the Treatise on Smallpox and Measles, in which he gave the first clear descriptions of smallpox and measles In the 1020s, Avicenna wrote The Canon of Medicine, in which he discovered the contagious nature of infection disease such as tuberculosis and sexuallity transmitted disease, and their distribution through bodily contact or through water and soil ; stated that bodily secreation is contaminated by "foul foreign earthly bodies" before being infected; and introduced the method of quaratine a means of limiting the spread of contagious disease.
When the Black Death bubonic plague reached al-Andalus in the 14th century, IBn Khatima discovered that infectious diseases are caused by microorganism which enter the human body. Another 14th century Andalusian physician, Ibm al- khatib (1313-1374), wrote a treatise called On the Plague, in which he stated how infectious disease can be transmitted through bodily contact and "through garments, vessels and earrings." In 1717, Mary Montagu, the wife of an English ambassador to the Ottoman Empire , observed local women inoculating their children against smallpox. In the late 18th century, Edward Jenner observed and studied Miss Sarah Nelmes, a milkmaid who had previously caught compox and was found to be immune to smallpox , a similar, but devastating virus. Jenner developed the smallpox vaccine based on these findings. After lengthy vaccination campaigns, the World Health Organization (WHO) certified the eradication of smallpox in 1979.
In the late 19th century Charles Chamberland developed a porcelain filter with pores small enough to remove cultured bacteria from their culture medium.Dimitri Ivanouski used this filter to study an infection of tobacco plants, now known as tobacco mosaic virus. He passed crushed leaf extracts of infected tobacco plants through the filter, then used the filtered extracts to infect other plants, thereby proving that the infectious agent was not a bacterium. Similar experiments were performed by several other researchers, with similar results. These experiments showed that viruses are orders of magnitudes smaller than bacteria. The term virus was coined by the Dutch microbiologist Marinus Beijerinck who showed, using methods based on the work of Ivanovski, that tobacco mosaic disease is caused by something smaller than a bacterium. He coined the Latin phrase "contagium vivum fluidum" (which means “soluble living germ”) as first the idea of the virus.
In the early 20th century, Frederick Twort discovered that bacteria could be infected by viruses. Felix d' herelle , working independently, showed that a preparation of viruses caused areas of cellular death on thin cell culture spread on agar. Counting the dead areas allowed him to estimate the original number of viruses in the suspension. The invention of Electron microscopy provided the first look at viruses. In 1935 Wendell Stanley crystallised the tobacco mosaic virus and found it to be mostly protein. A short time later the virus was separated into protein and nucleid acid parts. In 1939, Max Delbruck and E.L. Ellis demonstrated that, in contrast to cellular organisms, bacteriophage reproduce in "one step", rather than exponentially.
A major problem for early virologists was the inability to propagate viruses on sterile culture media, as is done with cellular microorganisms. This limitation required medical virologists to infect living animals with infectious material, which is dangerous. The first breakthrough came in 1931, when Ernest William Goodpasture demonstrated the growth of influenza and several other viruses in fertile chicken eggs. However, some viruses would not grow in chicken eggs, and a more flexible technique was needed for propagation of viruses. The solution came in 1949 when John Franklin Enders, Thomas H. Weller and Frederick Champman Robbin together developed a technique to grow polio virus in cultures of living animal cells. Their methods have since been extended and applied to the growth of viruses and other infectious agents that do not grow on sterile culture media.
