Biological Viruses: All Time Enemies

First came fever. Then Hamid Mansaray, a young nurse's aide at a remote African hospital, began to hemorrhage. Blood erupted from his nose and mouth. It burst out of capillaries beneath his skin and eyes.
By the time I reached the village of Panguma in Serria Leone, Mansaray lay isolated in a special ward. Doctors had diagnosed an obscure illness called Lassa fever. Its cause was a virus, an infective agent so small that 100,000 of them clumped together would still scarcely be visible. Viruses are little more than bundles of genes - strands of DNA or RNA, the molecules that carry the blueprints for all life. Yet viruses are far from simple. They invade are cells, causing ailments such as the common wart, as irritating as a cold, or as deadly as this bloody African fever (Jaret, pp. 64).
Viruses attack the body by taking over the cells of the body itself, some can be defeated by the body's white blood cells alone, but for others a cure is yet to be found.
Viruses are obligate intercellular parasites, particles composed of genetic material (DNA or RNA, but not both) surrounded by a protective protein coat. Outside a host cell, they are inert; inside, they enter a dynamic phase in which they replicate, pirating the host cell's enzymes, nucleic and amino acids, and machinery to accomplish what they are not equipped to do alone. Viral replication is often carried out at the expense of the host: diseases such as herpes, rabies, influenza, some cancers, poliomyelitis, and yellow fever are of viral origin. Of the estimated 1000 to 1500 types of viruses, approximately 250 cause disease in humans (over 100 of which cause the common cold), and 100 infect other animals (Microsoft Encarta 96 Encyclopedia).
For these reasons and many more Virus, fittingly, is derived from the Latin word for poison. Viruses are very simple in structure, consisting only of genetic material surrounded by a protective coat. The name was originally used in the 1890s to describe things that caused diseases but were smaller than bacteria. Viruses on their own are actually practically dead, but when associated with a living cell they can replicate many times, most of the time harming its host in the process. There are hundreds of known viruses that cause a very wide range of diseases not only in humans, but also in animals, insects, bacteria, and plants.
The existence of viruses was established in 1892. A Russian scientist named Dimitry I. Ivanovsky discovered what was later to be known as the tobacco mosaic virus. However the name virus was not used to describe these infectious particles until 1898 by a Dutch botanist named Martinus W. Beijerinck. Shortly thereafter viruses were found growing in bacteria, and later named bacteriophages.

(Microsoft Encarta 96 Encyclopedia)
T4 Bacteriophage:
This colored transmission electron micrograph shows a T4 bacteriophage, a virus that infects only bacteria (and in this case only Escherichia coli). Phages lack any reproductive machinery and rely on the apparatus of bacteria in order to replicate. They do so by attaching to the cell wall of the bacterium with the spidery tail fibers visible here. The tail is a sheath that contracts to inject the contents of the head, the genetic material (DNA), into its host. Within 25 minutes of infection, the bacterial apparatus successfully commandeered, viral progeny fill the cell. The overcrowded bacterium bursts, releasing approximately 100 new copies of the bacteriophage (Microsoft Encarta 96 Encyclopedia).
Then, in 1935, an American biochemist by the name of Wendell Meredith Stanley crystallized the tobacco mosaic virus and discovered that it was actually composed of the genetic material ribonucleic acid, or RNA. By the 1940s viruses had still yet to be seen, but this was made an actuality with the development of the electron microscope. This was followed quickly by the development of high-speed centrifuges which were used to concentrate and purify viruses. The study of animal viruses reached a major point in the 1950s when methods were developed to culture cells that could support virus replication in test tubes. By this method numerous viruses were discovered, and in the 1960s and 1970s most were analyzed to determine their physical and chemical characteristics.
Viruses undergo two different cycles of reproduction. The Lytic cycle and the Lysogenic cycle.

(Microsoft Encarta 96