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.