Virus is a microscopic organism that lives in a cell of another living thing. Although viruses are extremely small and simple, they are a major cause of disease. Some viruses infect human beings with such diseases as measles, influenza, and the common cold. Others infect animals or plants, and still others attack bacteria. Viruses produce disease in an organism by damaging some of its cells. However, viruses sometimes live in cells without harming them.

Viruses are so primitive that many scientists consider them to be both living and nonliving things. By itself, a virus is a lifeless particle that cannot reproduce. But inside a living cell, a virus becomes an active organism that can multiply hundreds of times.

Viruses are shaped like rods or spheres and range in size from about 0.01 to 0.3 micron. A micron is 0.001 millimeter or 1/25,400 inch. Most viruses can be seen only with an electron microscope, which magnifies them by thousands of times. The largest virus is about 1/10 as big as a bacterium of average size.

The study of viruses began in 1898, when a Dutch botanist named Martinus Beijerinck realized that something smaller than bacteria could cause disease. He named this particle a virus, a Latin word meaning poison. In 1935, Wendell M. Stanley, an American biochemist, showed that viruses contain protein and can be crystallized. This research and other studies led to the development, in the 1950's, of vaccines for measles, poliomyelitis, and other diseases. Virologists (scientists who study viruses) demonstrated in the early 1900's that viruses can cause cancer in animals. In the 1980's, research linked viruses to a few cancers in humans.

The structure of a virus. Viruses, unlike other organisms, are not made up of cells. Therefore, they lack some of the substances needed to live on their own. To obtain these substances, a virus must enter a cell of another living thing. It then can use the cell's materials to live and reproduce.

A typical virus has two basic parts, a core of a nucleic acid and an outer coat of protein. The core consists of either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). The DNA or RNA enables the virus to reproduce after it has entered a cell. Some RNA viruses contain an enzyme called reverse transcriptase, which converts virus RNA to a DNA copy inside cells. Such viruses are called retroviruses. The virus that causes AIDS (acquired immunodeficiency syndrome) is a retrovirus. The coat of a virus consists of individual proteins that give the virus its shape. This coat protects the nucleic acid and helps the DNA or RNA get inside a cell. Some viruses have an additional outer membrane that provides further protection.

How a virus infects an organism. Most viruses reproduce in specific cells of certain organisms. For example, viruses that cause colds reproduce in cells of the human respiratory tract. Viruses cannot live outside their particular cells. They must be carried into the organism by air currents or some other means, and then transported by body fluids to the cells.

When a virus comes into contact with a cell that it can enter, it attaches itself to the cell at areas called receptors. Chemicals in the receptors bind the virus to the cell and help bring it or its nucleic acid inside. The nucleic acid then takes control of the cell's protein-making process. Previously, the cell made only the proteins specified by its own genes. The genes are the cell's hereditary structures, and they consist of nucleic acid. A cell that has been infected by a virus begins to produce the proteins that are called for by the nucleic acid of the virus. These proteins enable the virus to reproduce itself hundreds or thousands of times.

As new viruses are produced, they are released from the cell and infect other cells. The new viruses become lifeless as soon as they are released. But they return to life after entering another cell. The viruses then start to reproduce and thus spread infection to more cells.

When a virus reproduces, it changes a cell's chemical makeup. This change usually damages or kills the cell, and disease results if many cells are affected. Some viruses change a cell only slightly because they do not reproduce. The DNA copy of a retrovirus may hide inside a cell on cell DNA. Such a virus may cause no immediate symptoms but might later damage the cell.

Virus diseases in human beings include AIDS; chickenpox; colds; cold sores; hepatitis, a liver disease; influenza; measles; mumps; poliomyelitis; rabies; and yellow fever. The nature of the disease caused by a particular type of virus is determined by which cells and tissues in the body the virus tends to invade.

The body protects itself from viruses and other harmful substances by several methods, all of which together are called the immune system. For example, white blood cells called lymphocytes provide protection in two ways. Some lymphocytes produce substances called antibodies, which cover a virus's protein coat and prevent the virus from attaching itself to the receptors of a cell. Other lymphocytes destroy cells that have been infected by viruses and thus kill the viruses before they can reproduce. However, some viruses are able to suppress the functioning of the immune system and thus enable themselves to reproduce more easily. Such viruses include those that cause measles, influenza, and AIDS.

Lymphocytes do not start to produce antibodies until several days after a virus has entered the body. However, the body has additional methods of fighting virus infections. For example, the body produces a high fever to combat such virus diseases as influenza and measles. The high fever limits the ability of the viruses to reproduce. To fight colds, the body forms large amounts of mucus in the nose and throat. The mucus traps many cold viruses, which are expelled from the body by sneezing, coughing, and blowing one's nose. The body also makes protein substances called interferons that provide some protection against many types of viruses.

The treatment of a virus disease consists mainly of controlling its symptoms. For example, physicians prescribe a drug called acetaminophen to bring down a high fever. In most cases, doctors cannot attack the cause of the disease itself, because most drugs able to kill or damage a virus also damage healthy cells. The U.S. Food and Drug Administration has approved a few drugs-including zidovudine (formerly called azidothymidine and commonly known as AZT), adenine arabinoside (ara-A), and acyclovir-for limited use against certain virus diseases. Researchers have found other potential antiviral drugs, including interferons. But these drugs must undergo further testing before their safety and effectiveness are known.

A few viruses are called slow viruses because they reproduce more slowly than the others. Some researchers believe a slow virus causes multiple sclerosis, a disease of the brain and spinal cord. Other viruses, such as the herpesviruses, can remain dormant in cells for years and then become reactivated and cause sporadic outbreaks of symptoms. Still other viruses including HIV, the virus that causes AIDS, can cause prolonged, persistent infections in which the virus multiplies continuously. Some viruses have been linked to human cancer. For example, hepatitis B virus is linked to hepatoma, a type of liver cancer. Burkitt's lymphoma, a cancer of the lymph tissues, may be caused by the Epstein-Barr virus. In addition, some leukemias are caused by human retroviruses.

Virus diseases in animals. Viruses cause hundreds of diseases in animals. These diseases include distemper in dogs and foot-and-mouth disease in cattle. Most virus diseases in animals occur in certain species. But some of the diseases spread to other species, and a few of them infect human beings. For example, dogs can give people rabies, which destroys nerve cells.

Certain viruses can cause cancer in animals. These viruses do not destroy all the cells they infect. Some of the infected cells have their chemical makeup altered, which causes them to behave abnormally. These altered cells reproduce in an uncontrolled manner, forming masses of tissue called tumors. Cancerous tumors invade and damage surrounding healthy tissue. Researchers have discovered a similarity between some viruses that cause cancer in animals and certain viruses that infect human beings.

Virus diseases in plants. Viruses infect all kinds of plants and can cause serious damage to crops. Plant cells have tough walls that a virus cannot penetrate. But insects penetrate the cell walls while feeding on a plant and thus enable viruses to enter. Plant viruses may infect one or two leaves or an entire plant. They produce billions of viruses, which are then carried to other plants by insects or air currents. Common diseases that are caused by plant viruses include tobacco mosaic and turnip yellows mosaic.

Viruses that attack bacteria are called bacteriophages. The word bacteriophage means bacteria eater. Bacteria, like plants, have tough cell walls. To penetrate these walls, most bacteriophages have a structure that works like a hypodermic needle. This structure consists of a sphere-shaped head that contains a nucleic acid, and a hollow, rod-shaped tail made of protein. When a bacteriophage enters a bacterium, the tail first penetrates the cell wall. Then the nucleic acid in the head moves through the tail and into the cell.

How viruses are used. Virologists study viruses chiefly to learn how they cause disease and how to control these organisms. Scientists also use viruses for such purposes as (1) insect control, (2) cell research, and (3) development of vaccines and other drugs.

Insect control. Certain viruses cause fatal diseases in insects. Virologists are seeking ways to use these viruses to kill insects that damage crops. The use of such viruses may someday replace insecticides, which kill insects but also may harm plants as well as other animals.

Cell research. Viruses are such simple organisms that scientists can easily study them to gain more knowledge about life itself. Research on bacteriophages has helped biologists understand genes, DNA, and other basic cell structures. Future research may provide further knowledge of how cells function and reproduce.

Development of vaccines and other drugs. Scientists produce vaccines from either dead or live viruses. Those used in dead-virus vaccines are killed by chemicals and injected into the body. For live-virus vaccines, virologists select very mild forms of living viruses.

Following is the table of some of the viruses which are responsible for many common human diseases such as colds, flu, diarrhoea, chicken pox, measles, and mumps. Some viral diseases such as rabies, haemorrhagic fevers, encephalitis, polio, yellow fever, and acquired immunodeficiency syndrome (AIDS) can result in death. German measles and cytomegalovirus can cause serious abnormalities or death in unborn infants. Of the estimated 1000 to 1500 types of viruses, approximately 250 cause disease in humans.

            Family                     Virus                                     Disease

Antiviral drug is the term for a group of chemical compounds used to treat diseases caused by viruses. Antiviral drugs usually do not cure viral diseases but can shorten the duration of the disease and lessen the severity of symptoms. Many antiviral drugs, however, can cause side effects, such as anemia or kidney damage.

Many viral diseases can be prevented through the use of vaccines. But vaccines have not been developed for all viral diseases, and vaccines are not useful for treating people once they become ill. Other drugs, including antibiotics such as penicillin, have no effect on viruses.

Viruses use substances within the cells of living organisms, called host cells, to manufacture enzymes and other materials they need to reproduce. Antiviral drugs work by interfering with parts of the viral life cycle that are different from steps completed by the hosts. This allows drugs to attack viruses while not harming host cells. However, since viruses rely on substances made by hosts to carry out many steps in their life cycles, there are only a few viral materials that the drugs can target. Getting drugs into infected cells is also an obstacle. In spite of these difficulties, researchers have developed many successful antiviral drugs.

Many antiviral drugs are chemical compounds that bind to viral enzymes, changing the structure so the enzymes cannot be used by the viruses. The drugs bind only to viral enzymes, while enzymes used by the host cell are not impaired. The virus, however, cannot reproduce to infect other host cells. The progression of the disease is thus slowed or halted by the drugs.

The first antiviral drugs were developed in the 1960's. One of the earliest, acyclovir, is widely used to treat infections of herpesviruses, a group of viruses that cause chickenpox, mononucleosis, shingles, and cold sores. This drug mimics a building block of the genetic material DNA (deoxyribonucleic acid) which herpesviruses need to reproduce. Viral enzymes mistakenly add this drug into a growing strand of DNA and stop its production. Acyclovir is given to patients by injection, as a pill, or in ointment applied to the skin. Other drugs, such as zanamivir, used to treat influenza, can be inhaled. Many antiviral drugs available today have been developed to treat HIV, the virus that causes AIDS.

Scientists are developing new antiviral drugs as they learn more about the structure and life cycle of viruses. Researchers analyze the chemical structure of viral enzymes and proteins. They use computers to design chemical compounds that will bind to the viruses without causing side effects in the host.

References:MS Encarta Encyclopaedia.World Book 2003