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: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 Encyclopedia)Lytic and Lysogenic Cycles of a BacteriophageAll bacteriophages (viruses that parasitize bacteria) have a Lytic or infectious cycle, in which the virus, incapable of replicating itself, injects its genetic material into a bacterium. By pirating its host’s enzymes and protein-building capacities, the virus can reproduce and repackage, making about 100 new copies before it bursts from and destroys the bacteria.
Some bacteriophages, however, behave differently when they infect a bacterium. The injected genetic material instead integrates itself into its host DNA, passively replicating with it to be inherited by bacterial daughter cells. In about 1 in 100,000 of these Lysogenic cells, the viral DNA spontaneously activates and starts a new Lytic cycle (Microsoft Encarta 96 Encyclopedia). Viruses are submicroscopic intracellular parasites that consist of RNA or DNA (deoxyribonucleic acid), never both, and of a protective coat of protein combined with lipid or carbohydrate components.
There is usually only one molecule of the nucleic acid, either singly or doubly stranded. Although some may have a nucleic acid that is segmented into two or more pieces. The protein shell is usually called the capsid, and the subunits of the capsid are called capsomers. Together these form the nucleocapsid. Yet other viruses have an additional envelope that is usually acquired as the nucleocapsid buds from the host cell. The complete virus particle is given the name virion.
Viruses are also obligate intracellular parasites, which means that their replication can only take place in actively metabolizing cells. When not in a live cell, viruses exist as inactive macromolecules. The most common of all viruses is the cold. At least four very different groups of viruses cause the symptoms we call a cold, Adenoviruses, Coronaviruses, Myxoviruses. Rhinoviruses.
Moreover, each of those has many varieties. More than a hundred distinct types of rhinovirus alone have been identified (Jaret, pp. 75). The common cold is an infectious disease of the upper respiratory tract, caused by more than 100 kinds of viruses. The infection affects the mucous membranes of the nose and throat, causing such symptoms as nasal congestion and sneezing, sore throat, and coughing. These symptoms are also characteristic of respiratory infections caused by bacteria, and of allergic reactions to things such as such as hay fever and asthma; therefore, the common cold is difficult to diagnose with certainty.
“Most adults catch between two and three colds per year” (Jaret pp. 76). There still is no treatment for the ‘common cold’, but many colleges and laboratories are working on it. A few of the less common but still very active viruses are, The chicken pox, otherwise known as varicella. (Microsoft Encarta 96 Encyclopedia)Chicken Pox Virus:The spherical varicella-zoster virus (VZV) infects most children worldwide by the age of 10. Transmitted in airborne droplets exhaled from an infected person, the virus causes a low fever and a rash of fluid-filled blisters that begin as red spots covering most of the body and the inside of the mouth.
The disease is dangerous to newborns, to people first infected in adulthood, and to those in whom the virus remains dormant in nerve cells, erupting as the more painful and sometimes chronic zoster (shingles) later in life. VZV is a member of the Herpes virus family, which also includes the causative agents of infectious mononucleosis, roseola, and oral and genital herpes (Microsoft Encarta 96 Encyclopedia). An extremely contagious viral disease, chiefly of children, characterized by early fever, an eruption of papules and vesicles, and mild constitutional disturbances. In most cases, fever is present 24 hours before the eruption appears. The eruption comes out on the face, scalp, or shoulders in crops of red, widely scattered vesicles, spreading slowly over the body, one crop maturing while another is appearing.
Pitting is rare, generally occurring on the face, where the lesions may become infected with germs from the environment. Quarantine is recommended for seven days after the appearance of the rash (Microsoft Encarta 96 Encyclopedia). Then there is there ever present influenza. Influenza is an infectious, highly contagious disease of the respiratory tract, especially the trachea, sometimes called flu or, less often, grippe.
The symptoms of a simple attack include dry cough, sore throat, nasal obstruction and discharge, and burning of the eyes; more complex cases are characterized by chill, sudden onset of fever, headache, aching of muscles and joints, and occasional gastrointestinal symptoms. In uncomplicated cases, symptoms fade and temperature drops to normal in a few days; the risk of death increases if the disease is accompanied or followed by viral pneumonia or bacterial pneumonia. Other common types of viruses are Hepatitis A, B, C, and E. Hepatitis A was previously known as infectious hepatitis, this disease is transmitted by contaminated food or other objects taken into the mouth, or it can be contracted by injection with improperly sterilized hypodermic needles.
Outbreaks often occur in army camps and in institutions where small children are crowded together. (Microsoft Encarta 96 Encyclopedia)Hepatitis B VirusThe hepatitis B virus (HBV) causes inflammation of the liver. The virus is recognizable under magnification by the round, infectious “Dane particles” accompanied by tube-shaped, empty viral envelopes. Manifestations of this condition include jaundice and a flu-like illness, while chronic infection can lead to serious pathologies such as cirrhosis and cancer of the liver (Microsoft Encarta 96 Encyclopedia). Hepatitis B, previously known as serum hepatitis, has only been recognized since World War II.
It is epidemic in parts of Asia and Africa. Hepatitis B is transmitted by injections transporting a virus-bearing serum, most often during blood transfusion, and by contaminated needles and syringes. For a person who has been accidentally stuck by a needle contaminated with the virus, administration of gamma globulin containing antibodies to the virus greatly reduces the chance of contracting the illness. The virus is also present in other body fluids and can be transmitted by sexual contact. In 1965 B.
Blumberg, an American physician, found a viral component called the Australia antigen that determines whether a sample of blood can transmit hepatitis B. All samples of blood intended for transfusion are now routinely tested for the antigen; this has greatly reduced post-transfusion hepatitis. Hepatitis C and D are considered “Non-A, and Non-B” Hepatitis. Hepatitis C, transmitted in blood or body fluids and caused by a virus which has now been cloned, is the most common cause of post-transfusion hepatitis.
Hepatitis E is transmitted in impure drinking water and can cause an epidemic form of non-A, non-B hepatitis. Another common virus is rabies. (Microsoft Encarta 96 Encyclopedia)Rabies Virus:The rabies virus is usually transmitted to humans by a bite from an infected dog, but the bite of any animal (wild or domestic) is suspect in an area where rabies is present. In North America, skunks are the principal carriers of the disease, although the raccoon and bat populations are also affected.
Symptoms of the disease appear after an incubation period of ten days to one year and include fever, breathing difficulties, muscle spasms, and in later stages, an irrational fear of water. Death almost invariably occurs within three days to three weeks of the onset of symptoms. For this reason, the emphasis of treatment is on prevention. In the United States, domestic dogs are vaccinated yearly and stray dogs are killed (Microsoft Encarta 96 Encyclopedia). Rabies is an acute, contagious infection of the central nervous system, caused by a specific virus that enters the body through an animal bite.
All warm-blooded animals are vulnerable, but in North America the disease is most common in skunks, foxes, bats, raccoons, dogs, and cats. Most of the cases of rabies in humans are caused by the bite of one or another of these animals. The infection period in humans varies from three weeks to 120 days, with an average of about four to six weeks. Rabies is almost always fatal when a vaccine is not given. There are even smaller infective agents than viruses called viroids.
Viroids are tiny infectious particle that causes disease in higher plants. Viroids are less than one-tenth the size of the smallest known viruses. Unlike viruses, which can contain either RNA or DNA, viroids consist solely of RNA. Viroids are even more different then viruses by their lack of a capsid.
Although viroids can be transmitted from one plant generation to the next and, by means of farm implements, from one plant to another, their method of replication inside the cells of a plant is not understood. Some of the symptoms are, infected plants show slowed or stopped growth and discoloration and may eventually die. As long as viruses are still attacking humans and other organisms, scientists will be constantly looking for cures. Bibliography”Ebola Virus Hemmorrhagic Fever: General Infection”. [online]. Available http://www.
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