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Virus, a creature-like being

It is the smallest creature in the world, arguably the most formidable, sentient enemy other than man himself. Before the beginning of the 20th century, hardly anyone had thought of the existence of this little thing, but now we are all familiar with its name – virus (virus). It is a Latin word meaning toxin or venom.

This smallest and simplest living organism was first discovered by the Russian botanist Dmitri Ivanovsky and the Dutch botanist Martinus Beyerlinck in the late 19th century while studying tobacco mosaic disease. The emergence of this “new” life form solved many of the medical problems of the early 20th century and refined Pasteur’s theory of bacteria. It made the scientific community realize that viral attacks were the cause of many human diseases and had to be studied and distinguished from bacteria.

This little thing was once very confusing: it has no cellular structure, no metabolic mechanism of its own, and no life activity in the usual sense when it exists alone, but it contains molecules (DNA or RNA) that store life information, has characteristics of life forms such as proliferation and heredity, and is able to evolve in competition. It survives by infecting host cells, using them as a cheap production plant to reproduce itself in large numbers without much effort and to screw up the cells and even the host.

Before the discovery of viruses, the scientific community actually had a basic understanding of life, namely that living organisms can survive, grow and reproduce through their own metabolism. The entry of viruses into the limelight sparked a debate about what viruses really meant for life. At the dawn of the era of molecular biology, most biologists tended to believe that viruses were only a form of existence resembling living things, not a true life form, and that they lacked some of the important genes needed to be a complete life.

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It is the world’s smallest living creature, can be called the most fearsome, sentient enemy other than humans themselves.

Viruses still leave a lot of mysteries to be solved and are closely related to human health and diseases. For example, where do pathogens come from? Why does nature tolerate the existence of those evil microorganisms? What factors determine the pathogenicity of microorganisms? These are the questions raised by J. Michael Bishop, an American microbiologist and winner of the 1989 Nobel Prize in Physiology or Medicine.

These are questions that deal with the origin and evolution of pathogenesis. A brief answer given by Bishop himself is that, for one thing, pathogens are never solitary and specific in the microbial community. They are either relatives of harmless microorganisms that have gone astray, or they cause harmless infections in one host, but are transferred to other “accidental” hosts and cause trouble. Second, from the time humans began domesticating animals and living in dense herds, they ran into trouble with the microbial community, and many fatal human infections apparently came from domesticated animals, and then spread for a long time through the chain of infection provided by village herds. Third, the pathogenicity of microorganisms is sometimes developed by suffering from foreign invasion. For example, the gene that causes cholera by producing toxins was brought to the cholera bacteria by a virus and then assimilated into the genetic structure of the bacterial host and fixed. Fourth, pathogenicity may be beneficial to microorganisms. Many of the adverse consequences of microbial infections appear to accelerate the spread of microorganisms from one host to another.

From the microbial perspective, death of the infected host is neither necessary nor even desirable. Bishop points out that the longer the host lives after infection, the more opportunities there are for microbes to multiply and spread. It is therefore reasonable to assume that over time, natural selection might slow the consequences of host infection. In fact, whether the virulence increases or decreases, almost any change in it is a response to natural selection.

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