THE EXTRACT FROM A BOOK

Ex 2.22 Read the extract from a book . Check the meaning of the words in bold in the glossary (Appendix 2) if necessary.

 

CLASSIFICATION OF VIRUS

 

Virology is the study of viruses and virus-like agents: their structure, classification and evolution, their ways to infect and exploit cells for virus reproduction, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is often considered as a part of microbiology. A major branch of virology is virus classification. Viruses can be classified according to the host cell they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages (viruses infecting bacteria, which include the most complex viruses). Another classification uses the geometrical shape of their capsid (often a helix or an icosahedron) or the virus's structure (e.g. presence or absence of a lipidenvelope). Viruses range in size from about 30 nm to about 450 nm, which means that most of them cannot be seen with light microscopes. The shape and structure of viruses has been studied by electron microscopy, NMR spectroscopy, and X-ray crystallography.

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria. Since the initial discovery of the tobacco mosaic virus in 1898, about 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth. Virus particles (known as virions) consist of two or three parts: the genetic material made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects these genes; and in some cases an envelope of lipids that surrounds the protein coat when they are outside a cell. The average virus is about one one-hundredth the size of the average bacterium. Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, influenza, herpes, polio, rabies and AIDS are examples of viral diseases. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines. However, some viruses including those causing AIDS and viral hepatitis evade these immune responses and result in chronic infections. Antibiotics have no effect on viruses, but several antiviral drugs have been developed. The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids - pieces of DNA that can move between cells - while others may have evolved from bacteria. The evolution of viruses, which often occurs in concert with the evolution of their hosts, is studied in the field of viral evolution. While viruses reproduce and evolve, they don't engage in metabolism and depend on a host cell for reproduction. The often-debated question of whether they are alive or not is a matter of definition that does not affect the biological reality of viruses.

Bacteria are tiny living things. Viruses are quite different, so much so that they blur the distinction between what is alive and what is just a complex chemical. A virus consists of at least two main components: a biological information molecule - either DNA or RNA - on the inside and a viral protein coat, or capsid, on the exterior. A few varieties of virus include a third component called an envelope, which surrounds the capsid. Think of a virus as a nanometer-scale syringe or hypodermic needle. The "syringe" is the viral protein coat, which is a complicated structure that usually has a geometric shape. Its function is to infect a host cell by injecting or otherwise inserting the DNA or RNA into that cell, where it mingles with the DNA and RNA of the host. The information molecules of the virus contain the blueprints for building more identical viruses. Once inside the cell, the viral DNA or RNA hijacks the cell's own molecular machinery for building proteins and forces it to makes copies of the virus, thus effectively converting the cell into a virus factory. The virus may also cause its host cell to make toxins. None of this activity is good for the cell, which usually dies, sometimes bursting in the process to release lots of new copies of the virus. As the human immune system cleans up the dead cells and responds to the virus, it produces inflammation and other symptoms. Many viruses can block their hosts from mounting an effective defense, and some actually trick the host's immune system into attacking healthy cells.

Viruses differ from bacteria in many fundamental ways that matter to food safety. Unlike bacteria, which can increase their numbers dramatically on or in food - even precooked food-viruses can reproduce only within the cells of living hosts. So viral contamination levels, at worst, remain constant in prepared food or ingredients; the contamination does not increase over time. Even though viruses do not reproduce independently the way that bacteria do, they do reproduce in a parasitic way, so they are subject to natural selection. They co-evolve with their host species and, over time, become quite specialized. Although most viruses infect just a single species, some adapt and cross over to infect other species. The rabies virus, for example, can infect most mammals, including humans. Meanwhile, the influenza virus can infect humans and a few other animals - notably pigs and birds - and the West Nile virus can infect humans, birds, and horses, among other animals. Many viruses specialize in infecting human cells, and those that do are either neutral or pathological. Unlike bacteria, which sometimes benefit humans, no natural human viruses are known to be beneficial. Nearly all viruses that cause food borne illness are specialized to live in humans and do not infect plants or other animals. Perhaps the most important way in which viruses differ from bacteria is how they die. Because viruses aren't alive in the same way that bacteria are, you can't kill them: instead, you must inactivate viral pathogens. Refrigeration or freezing do not inactivate viruses, but heat can do so. The thermal inactivation curve for a virus is very similar to the thermal death curve for bacteria. Like thermal death, thermal inactivation is an exponential phenomenon that depends on time and temperature. Unfortunately, much less is known about how heat inactivates viruses than about how heat kills bacteria. Unlike many bacteria, most viruses are hard to grow in a laboratory. The problem is particularly acute for food borne viruses that infect human gut cells; those cells can themselves be difficult and expensive to culture.

The noroviruses aptly illustrate the conundrum that many viral pathogens pose to science. Although noroviruses are among the most common food borne pathogens, thought to collectively cause more than nine million cases of food borne illness each year in the United States - and to sicken many millions more around the globe few details have emerged about the mysterious microbes. Noroviruses have been infecting humanity from time immemorial, yet they were unknown to science until an outbreak of foodborne gastroenteritis, or intestinal inflammation, in 1968, at a school in Norwalk, Ohio. Following that episode, related viruses were found in similar outbreaks worldwide. Microbiologists originally lumped the burgeoning group under the name Norwalk virus. They subsequently became known as Norwalklike viruses (NLVs) then, in 2002, were officially classified under the genus Norovirus. It took some 40 years after noroviruses were discovered for researchers to successfully cultivate the viral particles in a laboratory - a feat not accomplished until 2007. In the meantime, investigators learned what they could from genetic sequencing of noroviruses' viral RNA, epidemiological studies of infected humans, and research on related viruses that infect cats and mice. Noroviruses mainly sicken humans, and contamination occurs chiefly via the fecal-oral route. Investigators of outbreaks have implicated foods, such as salad dressing, raspberries, sandwiches, and cake frosting, served in a wide range of places, from schools to cruise ships to some of the world's best restaurants. The viruses can also affect people who eat foods that were contaminated at the source, such as shellfish tainted by human feces.

Once inside a person, the pathogens can spread rapidly. A single virion (infectious virus particle) of  Norwalk virus is 50% likely to produce an infection - the highest infection rate of any known virus. No surprise, then, that specialists attribute half of all foodborne gastroenteritis outbreaks to noroviruses. Recovery from a norovirus infection does not produce long-term immunity, which means that people can get reinfected repeatedly. No one knows whether this lack of immunity is caused by diverse viral strains or by some other feature of the virus. Intriguing research, however, has indicated that people with certain blood types may be resistant to norovirus infection. Norovirus infections are generally mild and are only rarely lethal. Symptoms occur after a typical incubation period of one to two days and include nausea, vomiting, diarrhea, and abdominal pain; they usually ease in a few days. But viruses can be shed for as long as two weeks after recovery.