Virus

A virus is a package of chemicals that infects living cells. Composed only of some genetic material inside a protein coat, it is not considered a living thing since it does not reproduce on its own nor does it carry on respiration (the process by which living things obtain energy). A virus is only able to do these things while inside a host cell, which it usually kills in the process of duplicating itself, often causing a disease.

Ever since the existence of viruses was first demonstrated just before the twentieth century, viruses have regularly puzzled biologists who sought to classify them. Examining their characteristics seems to place viruses on the borderline between living and nonliving things. Viruses are smaller than the smallest bacteria, and were not able to actually be seen until the postwar invention of the electron microscope. Being able to see them and watch how they behaved did not make it any easier to classify them, however. Viruses at first appear to be a living organism since they are made of some of the same compounds and chemicals that are found in living cells. However, they are not themselves cells nor are they made up of cells (as all living things are). Furthermore, viruses do not have a nucleus or any other cell parts, and they cannot reproduce unless they are inside another living cell. In fact, they do not carry out any of the typical life processes, or metabolic activity, that living cells do. Despite all of this, when they find an appropriate cell, they quickly reproduce themselves, usually to the disadvantage of the host cell and sometimes the entire organism.

Anyone who has ever had a case of the chicken pox, measles, or the flu has been infected by a virus. Some viral diseases like polio, AIDS, yellow fever, meningitis, encephalitis, and rabies can be fatal. Others, like herpes or the common cold, only make us sick temporarily. Overall, viruses are responsible for at least 60 percent of the infectious diseases around the world. A virus can also infect bacteria and fungi as well as plant and animal cells. Viruses are incredibly tiny and come in different shapes according to what type of cells they invade. Animal viruses are usually round and look like little puffballs. Plant viruses resemble rods, while bacterial viruses look like tadpoles with little tails. Despite their appearance, they do not have a true cell structure with a nucleus or other parts of a cell. Instead, viruses consist of a core of nucleic acid covered by a protective coat of protein called the capsid. The nucleic acid (either DNA or RNA) contains directions for making more viruses.

Unlike a living cell, a virus has one job only—to reproduce or replicate itself. Once inside, it acts like a computer program for making new viruses. When it enters or infects a cell, a virus takes over the cell's metabolism or its chemical reactions, since it does not have the machinery, the energy, or the raw materials to do anything on its own. Like an invader, it takes charge of the cell and instructs it to produce everything that the virus needs to reproduce itself. In a sense, it "reprograms" the cell it invades so that the infected cell's systems and energy are available to and controlled by the virus.

Biologists have been able to study viruses in action by observing how they infect bacteria. First, viruses do not attack just any type of cell. Viruses do not cross kingdoms, so a virus that attacks a plant cannot infect an animal. Other viruses only attack certain species within a kingdom, while others will only attack certain types of cells in a species. Once a virus finds its proper cell type, it first attaches itself at a certain place on the surface of the cell. It next penetrates the cell by drilling into it, which immediately stops the cell from making its own genetic material.

With the virus DNA or RNA now inside the cell, the cell begins to go to work copying and making virus DNA. After a half-hour or so when the cell has made many copies (one polio virus can produce 100,000 copies of itself inside a single cell), it makes a certain enzyme called a lysozyme. This enzyme attacks its cell walls, which soon burst open, killing the cell but releasing the many virus copies. These go on to invade and take over more cells, all the while reproducing more and more of the virus. Some viruses do not destroy the cell but simply join their DNA to that of the cell which, when it divides, also reproduces the virus. The organism whose cells are infected by the virus becomes diseased either because the virus is destroying its cells or damaging those that it takes over.

Fighting a virus is very difficult. Drugs that work against bacteria cannot work on viruses since viruses have no metabolic activities (the internal processes that make a cell work) of their own to attack. Since viruses use all of the infected cell's systems, anything that would stop a virus from reproducing would also attack the cell itself. By destroying the virus, the cell is also destroyed. An organism's natural defenses are the best antivirus weapon, so virus-caused diseases can be prevented by the use of certain vaccines. A vaccine is a substance that contains a weakened version of a virus that, although it can no longer cause a disease, encourages the body to create substances called antibodies that will later specifically attack and kill the virus when it invades. Vaccines have proven successful for many virus-caused diseases like polio, measles, and the many different types of influenza.

Viruses have ways of fighting back, including mutating or changing their makeup. Viruses like the flu or HIV (the Human Immunodeficiency Virus that causes AIDS) are able to change frequently enough so that a certain type of vaccine that may have been successful will no longer work on the new version. Other viruses can stay hidden for a long time. This type of virus is called a latent virus, and it does not take control of the cell as soon as its penetrates it. It still duplicates itself but does not harm the cell until at some later time when it becomes active and begins its destruction.

Although modern medicine has made great strides in fighting or preventing viral infections, it has proven especially difficult to create new drugs that will limit or stop the growth of a virus inside a cell without doing the same to the cell itself. Ever since their discovery, viruses have proven to be not only a baffling phenomenon but also an ultimate type of parasite that has proven to be a potent and highly resourceful enemy.

[See alsoAIDS; Immune System; Immunization; Nucleic Acid]