Virus | Research & Encyclopedia Articles

Virus

Viruses are tiny particles that consist mainly of protein and nucleic acid such as DNA or RNA. Because they contain genetic material, they are able to reproduce, but lacking cell structures and the ability to metabolize food, they can reproduce only inside other living host cells. Viruses, far smaller than the smallest bacteria, can be crystallized, much like a mineral forms a crystalline structure. Thus, viruses are not considered alive. But because they undergo genetic recombination and mutation, viruses are very adaptable and are found throughout the biosphere, from the viruses that cause AIDS, chicken pox, and measles in humans to the viruses that live within plants and cause tobacco leaves to wilt and tulips to turn bright colors.

The existence of viruses was only hypothesized a little over a hundred years ago. In the late nineteenth century, Louis Pasteur, the notable French chemist and microbiologist, suggested that there might be pathogenic organisms smaller than bacteria. After much experience culturing bacteria, he turned his attention to rabies, a serious infectious disease caused by viruses and transmitted through the bites of rabid animals. In 1885, Pasteur weakened or attenuated the rabies germs by passing a rabies infection through different species of animals until it was less virulent. His preparation was complete when a nine-year-old boy was brought to him for treatment. The boy had been bitten and mauled by a mad dog, and Pasteur's preparation of attenuated germs successfully saved the boy's life. This triumph over rabies was the crowning achievement of Pasteur's career, yet the great scientist was perplexed at not being able to locate the germ that causes rabies. It was a virus--too small to see with his compound light microscope.

Simultaneously, one of Pasteur's associates, Charles-Edouard Chamberland (1851-1908), was working on a number of technical advances that made bacterial research more efficient. He introduced the autoclave, an airtight heating device that could be used for killing bacteria. He also fabricated laboratory filters, made out of unglazed porcelain, which were well-suited for filtering out bacteria. Chamberland's filters were used by Dmitri Ivanovsky, a Russian botanist who in 1892 was the first to demonstrate that some pathogenic agents smaller than bacteria were linked to disease. He crushed tobacco leaves that were infected with the tobacco mosaic disease, a disease that dwarfs tobacco plants and creates a mottled, mosaic pattern on their leaves. He filtered the sap released by the infected leaves, and when the filtrate was added to healthy plants, they, too, got the disease. Ivanovsky concluded that either tiny particles in the filtrate were responsible for transmitting the disease or there was something wrong with his filtration system. However, he mistakenly thought that anything that went through the filters must be minuscule bacteria.

Just six years later, a Dutch bacteriologist searching for the cause of tobacco mosaic disease, Martinus Willem Beijerinck, confirmed Ivanovsky's findings but concluded that the infection of tobacco mosaic disease is not caused by bacteria but by a much smaller virus--a term that Beijerinck coined. He believed the filtrate itself was the pathogen, and he called it a "filterable virus." Virus is a Latin word for poison. In that same year, another scientist, Friedrich August Löffler (1852-1915), showed that a virus caused foot-and-mouth disease in livestock.

It was not until the 1930s that the crystalline nature of viruses was discovered. Wendell Stanley (1904-1971), an American biochemist, prepared large quantities of tobacco mosaic virus, or TMV, by growing tobacco and infecting new plants in much the same way as his predecessors. However, during the 1930s, groundbreaking work in chemistry was being done by John Northrop and James B. Sumner, American chemists who had crystallized enzymes and found them to be made of protein molecules. Stanley put the filtrates containing the TMV through the same procedures used to crystallize proteins, because he hypothesized that viruses, too, were made of protein. In 1935, he achieved the crystallization of viruses, finding that the needle-like crystals have all the pathogenic properties of the virus. However, his findings were difficult for many to accept. The results suggested that the virus itself was not a living thing, just as an enzyme is in itself not alive. However, the virus does reproduce when within living cells. The discovery of something that straddled the border between living and nonliving stirred up arguments about what is alive and what is not that continue even today. For his work on crystallizing viruses, Stanley shared the 1946 Nobel chemistry prize with Sumner and Northrop.

In 1937, more details about the structure of the tobacco mosaic virus were uncovered by Sir Frederick Bawden (1908-), an English plant pathologist. After completing graduate work at Cambridge in 1930, Bawden began working on TMV. He found that tobacco mosaic viruses contain ribonucleic acid, or RNA. Nucleic acids are found in all viruses and thus give them one of the characteristics of living things--the ability to reproduce with genetic material. Ten years later, a group of American virologists isolated and studied several important viruses. Working at Children's Hospital in Boston, Thomas Weller began perfecting ways of culturing viruses, keeping host cells alive by changing the medium instead of transferring the cells. Work with viruses in the past had often been contaminated by bacteria, but in the 1940s, antibiotics made it possible to grow viruses free from bacterial contamination. Weller used this method to study the mumps virus and the virus that causes chicken pox. Along with John Enders and another colleague, Frederick C. Robbins, Weller cultured the poliomyelitis virus for many generations, making it possible to culture a weakened or attenuated virus that could multiply without being dangerous--the kind of virus that is ideal for a vaccine. This lab technique enabled Jonas Salk and Albert Sabin to later develop a polio vaccine and earned Weller, Enders, and Robbins the 1954 Nobel Prize for Physiology or Medicine. Weller also studied the herpesvirus that causes chicken pox and shingles as well as the rubella virus.

At the same time that many scientists were researching viruses, the search for the genetic material of the cell was also being pursued. The work of Heinz Fraenkel-Conrat showed that the nucleoprotein and not the protein part of a virus is the component responsible for viral activity. In 1955, Fraenkel-Conrat developed a technique in which he separated the outer protein coat from the inner nucleic acid core of TMV. He then put the two components back together again and demonstrated that the virus could still infect other cells. Through experiments, he showed that the protein part of the virus is inert, but the nucleic acid portion alone has the ability to infect cells. Research since the 1960s has uncovered numerous groups of viruses. The very largest, such as the virus that causes smallpox, are more complex in structure and contain double-stranded DNA. The smallest viruses, the picornaviruses, include the viruses that cause the common cold and polio. They are hundreds of times smaller than poxviruses and have a very geometric polyhedral shape. Other intermediate groups of viruses vary in size, shape, and the presence or absence of a tail or protective envelope.

Research in the last two decades has uncovered two other groups of particles even smaller than viruses that replicate only when inside a living cells. Viroids are very short single strands of RNA that enter a plant cell host and lodge in the cell nucleus, where they interfere in some way with cell function. Viroids are responsible for plant diseases that have ravaged certain crops as diverse as coconut trees in the Philippines and chrysanthemums in the United States. Another group of particles similar in behavior to viruses are prions, chains of glycoprotein that enter cells and can cause certain rare nervous disorders in humans and animals.

At the close of the twentieth century, outbreaks of several viruses captured public interest. In May 1993, for example, an outbreak of unexplained illness occurred in the "Four Corners" region of the American Southwest, an area shared by New Mexico, Arizona, Utah, and Colorado. A number of previously healthy young adults suddenly became acutely ill with respiratory symptoms, and about half soon died. Researchers tracked their illness to a form of hantavirus transmitted chiefly by the deer mouse. By 1998, hantavirus pulmonary syndrome had been identified in over half of U.S. states.

In 1995, the deadly Ebola virus struck in Zaire, setting off public panic. More than three-quarters of the over 300 victims died before the epidemic was contained. Then in 1997, an unusual influenza virus struck several people in Hong Kong. What made this outbreak different was that the infected people apparently caught the virus from chickens. Although mild bird viruses have infected people in the past, this was the first report of chicken-to-human transmission of a virulent strain of flu. Such incidents, and viral diseases such as AIDS, are the plagues of modern times.