Viruses are much smaller even than bacteria and can only be seen with an electron microscope. They range in size from about 10 to 330 millionths of a millimetre.
A virus organism is known as a micro-organism, they cannot grow or reproduce without a living cell. A virus invades living cells and uses their chemical makeup to keep themselves alive and to reproduce. Scientists believe they reproduce very well in the right conditions but are known to mutate.
This ability to mutate is responsible some viruses to be able to change slightly in each infected animal or person, Viruses can cause many common human infections and are responsible for a number of rare diseases. An example of a viral illnesses is the common cold.
Scientific and medical researchers have categorized viruses into several major families, and are based on their shape, behaviour, and other characteristics. Main categories that are on the DNA side include the herpes-viruses, papovaviruses (including the papillomaviruses), adenoviruses, poxviruses, hepadnaviruses, and parvoviruses.
On the RNA virus side, these major families include the picorna-viruses (including the rhinoviruses), calciviruses, paramyxoviruses, orthomyxoviruses, rhabdoviruses, filoviruses, and retroviruses. There is also many more smaller virus families within the major classifications. Viruses can be host specific, capable of infecting and causing disease in humans only or specific animals only.
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