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When it was discovered that bacteria could cause disease, there were many illnesses
for which bacterial agents could not be found. These included serious diseases
such as smallpox, polio, rabies and influenza as well as less serious maladies
such as the common cold. These diseases, and many more, are caused by viruses.
Viruses are infectious agents which are between 20 and 100 times smaller than bacteria. They range from the poliovirus, 20 nm in diameter, to smallpox virus, 250 nm in diameter. They are too small to be seen through a light microscope and can be visualized only by an electron microscope. Viruses consist of strands of genetic material (RNA or DNA) surrounded by a coating of protein (called a capsid). Viruses are not considered to be living organisms because they cannot reproduce outside of a living cell.
A virus binds to a cell when a part of the outer viral structure docks with a specific molecule on the cell surface. It then crosses the cell wall through a variety of methods (for example, injecting the genetic material through the cell wall or incorporating its capsid into the cell wall).
Viruses multiply by inserting their genetic information into a cell and harnessing the cell machinery to replicate the viral genetic material and make new capsids. The new viruses are then transmitted to other cells, either through the cell wall or when the cells burst and die. Viruses are parasites on a cellular level.
New strains of disease causing viruses are a constant threat because viruses mutate easily. Each year, a new influenza virus causes illness in millions of people. As a result and because most drugs cannot disable a virus without harming healthy tissue, it is difficult to develop drug therapies to cure viral infections.
Viruses can be transmitted from one animal species to another. They affect each species differently and an animal host can harbor viruses deadly to man such as the Ebola and Hanta viruses, without any ill effect. In the same manner, man can harbor viruses deadly to other animals. The Ebola-Reston virus is deadly to monkeys but does not cause illness in humans.
Viral illnesses can be spread in a number of ways. Some, such as AIDS and Ebola, can only be spread through exchanges of blood or other bodily fluids. Others, such as influenza and the common cold, are air borne and enter through the respiratory tract. The Hantavirus is spread by breathing dried rodent feces which become airborne.
* The ebola virus is in the shape of a long rod, usually 800 to 1000 nanometers long. (A nanometer is one billionth of a meter.)
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