The virus has derived its name from the Latin term 'virus' denoting toxin or poison and is an ultramicroscopic infectious agent that reproduces itself only inside the cells of living hosts. Many viruses are pathogenic or disease bearing microorganisms that can contaminate all types of organisms, including animals, plants, bacteria as well as archaea. While these microscopic organisms possibly existed since life began on earth, they were discovered only a few centuries back. In fact, in 1892, while Dmitry Ivanovsky, a Russian botanist, was engaged in examining tobacco mosaic disease that resulted in blemishes and swelling up of the tobacco leaves. During the course of his investigation, Ivanovsky allowed the juice obtained from contaminated tobacco plants to go through a porcelain filter that was supposed to enmesh all kinds of microorganisms, inclusive of bacteria and other most diminutive pathogens known to mankind. In fact, the porcelain filter had been developed by a trustworthy junior of Louis Pasteur named Charles Chamberlain and was extensively made use of domestically to filter as well as disinfect drinking water.
Ivanovsky was surprised to note that despite filtering the sap from the infected tobacco plants, they lead to contamination. This made him come to the conclusion that the filter was unable to trap the agent responsible for the contagion, which was eventually passing through strain. This made the Russian botanist wonder if any toxin or poison produced by bacteria was responsible for tobacco mosaic disease - the malady affecting the tobacco plants. His suspicion was on the basis of the fact that it had been found that certain human ailments like diphtheria were actually had their origin in certain powerful poisons produced by bacteria and these were able to go through the filters invented by Charles Chamberlain with no difficulty. Like Ivanovsky, several other scientists of the time held a similar opinion on this issue.
Unfortunately, Ivanovsky as well as the other scientists of the day were proved to be wrong. Around some six years after Ivanovsky carried out his investigation on tobacco mosaic disease, a Dutch botanist called Martinus Beijerinck picked up the research from where the Russian botanist finished. The Dutch botanist adopted a different approach - something very dissimilar to Ivanovsky's experimentations. Beijerinck used the sap collected from a diseased plant and sprinkled it on a healthy plant. Next, he collected the juice from the plant after it too became contaminated and continued the process passing the sap from the diseased plants to numerous tobacco plant generations. As a result of such consecutive jabs, the plants kept on developing the blemishes that are typical of the tobacco mosaic disease. In fact, even after innumerable plant passages, the sap of the contaminated tobacco plants preserve its belligerence in full strength. Such non-dilutable characteristic of the sap hinted that the agent responsible for the disease was replicating itself inside the plant. Thus, the concept that a toxin was responsible for the disease was eliminated. This was enough to give an idea that the agent causing the contagion was neither bacteria nor any toxin or poison. Hence, Beijerinck initially named the disease causing pathogen as a contagium vivium fluidum denoting a 'contagious living fluid'. However, afterwards he named the agent virus borrowing the term from Latin. As mentioned earlier, virus in Latin denotes a 'poison' or a 'poisonous substance'. Despite all this, at that time, nobody was actually aware of the precise substance they were coping with.
Although the scientists of that time were still not aware regarding the temperament or natural history of viruses, in a little while they discovered that this 'soluble living germ' was also responsible for a number of other maladies. In fact, scientists first discovered that the 'foot-and-mouth' disease prevalent in cattle was also caused by this contagious agent. They found that this ailment was caused by the infectious agent that was capable of passing through the finest filters available in those days and were much smaller compared to bacteria. In 1900, it was established that a human ailment known as the yellow fever was also caused by a germ that had its roots in virus. And, as of date, we are familiar with over 5,000 dissimilar types of viruses that are capable of contaminating any living organism.
Viruses are actually very obstinate microscopic organisms and what may come they will find some way or the other to enter the living cells to gain genetic advantage at our cost. According to past records, the viruses have been engaged in this task for several thousand years. The presence of small pox in ancient Egypt is evident from the blemished mummified face of Ramses V. Bas-relief (a type of structural relief) engravings done in that era also portray a priest having shrunk legs that indicates that polio also existed in ancient Egypt. And there is little doubt among scientists as well as historians that viral infections were very common in ancient Egypt. It is also believed that even the earliest man living in the caves too had to confront viruses. In fact, the manners wherein the viruses get inside the cells and take hold of control can only happen by means of a prolonged involvement that have helped the viruses to become accustomed to the host cells. In fact, even to this day, viruses are continually working to adapt themselves to their hosts.
However, it is only possible to guess the time from when such association actually started. One of the several theories on the subject says that viruses appeared earlier than the cells as they are much smaller and simpler compared to the cells. According to this assumption, the nucleic acids present in the viruses became more and more complicated before they turned out to be the material that comprise the cells. While this theory was widely accepted at one time, presently scientists think that this is a very implausible situation. In fact, there is another theory on this subject that appears to be more plausible. According to the second hypothesis, the viruses developed from fragments of the genetic substances present in the cells. These bits of genetic substances are believed to have run off from their cells. Advocators of the second hypothesis say that on due course, these 'run off' genetic materials or the fragments developed from the host nucleic acid acquired the aptitude to exist as independent parasites that reproduced by themselves within the cells. These 'escaped' genetic fragments eventually developed into viruses.
A renowned British scientist and astronomer, Fred Hoyle provided a third theory regarding the origin of viruses. According to Hoyle, the viruses actually fell on the earth from outer space and they continue to fall even to this day. However, most contemporary virologists do not concur to this view regarding the genesis of viruses. All said and done, no matter what may have been responsible for the origin of viruses, the fact remains that once the viruses have set themselves up on the earth, they became a force to be prepared for.
In the 1970s, during researched conducted on Rous sarcoma virus, scientists first became aware of the fact that, in addition to making us ill, the viruses were capable of doing more things. In fact, this virus is responsible for a fatal cancer in chickens. The virus causes cancer in the chickens by introducing a gene that causes cancer, known as oncogene, straight away into the DNA of the host cell by means of a procedure known as integration.
Gradually, scientists became inquisitive regarding this oncogene and started examining the microscopic organism more minutely. During the course of their researches, scientists were amazed to note that oncogene was a very familiar gene normally present in hale and hearty chicken. It appeared that the sarcoma virus was seizing this gene from the host cell and taking it to different cells in other chickens. While seizing the gene it came into close connection with viral genes and this transformed the typical chicken gene, converting it into a cancer provoking agent.
Despite having detected so much about oncogene, the gene continued to remain an alien to the scientists. It was unbelievable that the 'abducted' gene was found to be virtually omnipresent as it began to be found in an assortment of vertebrates, such as fish, cow, rats and even humans. And, possibly this omnipresent nature of the gene was not a happenstance. This was possibly owing to the fact that viruses were responsible for spreading this gene in chickens as well as other animals. Going by this theory, it may be rationally concluded that viruses would later emerge as agents for dispersing a complete host of genes. And if this happens, viruses would be graded on the upper side with sexual reproduction being a key power to result in genetic inconsistency and eventually the progression of different species.
A scientific discovery achieved in 1977 really astounded the entire genetics community. In that year, scientists researching oncogene happened to come upon introns in the genes of rabbits and chickens. It may be mentioned that an intron is an expanse of worthless DNA - DNA that does not code or send any signal for producing proteins and, till so far, has no perceptible role. Hence, it is not surprising that scientists talk about introns as junk DNA. Additional research on the topic led the scientists to discover the presence of introns in several other animals and most surprisingly, even humans. Scientists were also surprised to learn one truth - the better evolved an organism is the more number of meaningless DNA or introns it had. Although it may seem to be incredible, the fact is that over 95 per cent of the DNA in humans comprises useless expanses of introns or junk DNA.
Well, this made scientists wonder if the introns were actually the residual fragments of very old viral contagions, a remnant inheritance of the bygone days of our evolutions. Well, this is very much possible because ever since scientists discovered junk DNA, various researches undertaken by molecular biologists have shown that particular expanses of the human DNA has very close resemblance to the genomes of specific viruses. On the other hand, a number of viral DNA found in the humans are very much similar to the fragments found in the chimpanzees, the non-humans who are most intimately related to the humans. According to hypothesis put forward by the researchers, around 10 million or a hundred million years ago this virus entered the DNA of an ape that would later evolve into chimpanzees and humans. They also assume the existence of a virus during that period that might have resulted in a fatal epidemic - possibly an ancient form of AIDS. Now, the virus is something akin to an excess baggage present in the inheritor cells of living organisms those continued to live even after enduring its horror. In fact, the introns are now considered to be silent reminders of various prehistoric contagions.