Viruses Have been very important part of medical researches and tretment in the past years , however we will talk today about the history of it. I hope you find this article helpful
Yellow fever and smallpox leap to mind. Others remember sleepless nights while children suffered the discomforts of measles, mumps, chicken-pox or even the "common cold". Finally, there are those to whom it suggests the fundamental cause of cancer. To our ancestors the Latin word "virus" meant a slimy substance, poison or venom including the agents responsible for transmissable or infectious diseases. With the advent of the microscope in the nineteenth century microbes were discovered and divided into bacteria, protozoa and fungi. However, the word "virus" still meant "disease-producing" and when some members of these groups were found to be responsible for certain diseases they were, to all practical purposes, "virus" microbes.
Pasteur and Roux knew that rabies was a specific disease and although they could not see the "microbe" causing it they assumed, quite correctly, that this microbe was very small—but it was still a "virus" microbe. Nowadays, by definition, a virus is a submicroscopic filterable entity capable of self replication only in specific host cells1. The "filterable virus" came to light in 1892 when Ivanowsky reported that the juice from tobacco plants with mosaic disease remained infectious after passage through a filter, fine enough to hold back the smallest microbe then known. Nevertheless he too assumed that the tobacco mosaic "virus" was a small microbe. It was not until 1898 when Beijerinck repeated and confirmed this work that the idea of the "filterable virus" was introduced.2 He also found that the infectivity of the "virus" survived precipitation in ethanol and could diffuse through agar gels. Therefore he stated that this was not a microbe but a "fluid infectious principle". Needless to say most informed people of the time rejected this idea, but with the discovery that similar agents caused foot and mouth disease3 and yellow fever4 the way was clear for the "filterable viruses" or "ultraviruses" as they were then called. Today The Viruses are exclusively the "filterable viruses" and are considered apart from The Microbes. In 1957 Lwoff5 stated the differences between viruses and microbes and his conclusions have been generally accepted. Using Lwoff's work as a guide, the nature of viruses, their dissimilarity to the microbes and their similarity to genetic material can be simply outlined
With the exception of the last item on the table all the remaining characters are interrelated. For an organism to possess both DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) in addition to a full complement of enzyme systems for growth and free multiplication a certain internal volume is required. The size and obligate intracellular growth are only reflections of the constitution of the different forms of life. It is the actual make up of a virus which separates it from all other living organisms and which in fact places it nearer the gene in every sense. So far there is no direct evidence to state that RNA is like DNA in that the latter acts as the genetic messenger among the microbes and higher organisms but this will almost certainly be proved so—ultimately. We have, therefore, the intriguing possibility that viruses are "wild" or "mutinous" genes and ribosomes. This theory postulates that the DNA containing viruses are the result of a gene breaking away from the chromosome of the cell
It then uses its abilities to replicate itself and initiate protein formation to produce an entity capable of surviving the rigours of extracellular existence and of infecting and multiplying within specific host cells. It is but an extension of this theory to postulate that the RNA containing viruses are "mutinous" ribosomes. In direct opposition to this theory there is the "classical" or degenerative theory which postulates that viruses have evolved from bacteria by losing the enzyme systems necessary for extracellular growth and ultimately one or other of the nucleic acids. The supporters of this theory point to the gradual progression through the Rickettsiales which behave like minute bacteria, Psittacosis—L.G.V.—Trachoma which contain both RNA and DNA, to the largest of the "true viruses", the Poxvirus group
From a consideration of both these theories a third was naturally advanced. Lwoff stated that the genetic material of the bacteriophage and the genetic material of the bacterium both evolved from a common structure—the genetic material of a primitive bacterium.5 In the final analysis this must of course be the case, always providing one accepts evolution, because all genetic material evolved from a common structure—the first gene. Probably the viruses of the Poxvirus group evolved by degeneration from bacteria whereas the Picornaviruses and Papovaviruses evolved from mutinous ribosomes and genes respectively
From being obligate intracellular parasites it automatically follows that they must produce some change in the cell. Viral invasion of a cell may result therefore in the death of the cell or the transformation of the cell into something combining the properties of both cell and virus. Known transformations of host cells occur with the production of lysogenic bacteria or tumour cells. Destruction of the host cells in a multicellular organism is usually known as an illness, which may be subclinical or result in the death of the host. This type of infection is best demonstrated in humans by diseases such as poliomyelitis, yellow fever, smallpox, measles, chickenpox and the "common cold". It has still to be proved that human cancers are caused by viruses but this seems more than likely when one considers the nature of cancer and the known carcinogenic properties of certain viruses.6 Having established our viral diseases the question then arises—how do we cure them? At the present time the old adage is still the best Prevention is better than cure. As yet there is no cure for any of the virus diseases, despite the claims of many drug manufacturers—unless one adopts the mind over body position
To destroy the virus necessarily involves destruction of the infected cells and to do this without irreparably damaging the uninfected cells has so far proved impossible by chemotherapy. A substance, interferon, has been isolated from virus infected cells, which does inhibit infection of other cells, but to date there has been no really successful application of interferon.7 Prevention of disease can be effected by the immunisation of susceptible hosts to produce circulating antibodies which neutralise viral infectivity. This vaccination has been successfully applied to most of the diseases mentioned in this chapter and we will discover that the history of virus vaccines antedates the history of Viruses and Virology by exactly one century
