Virus are very small particles which may cause disease in man, animals, and plants. The words "particles" seems a stranger way to describe them, so let's see why it used.
Virus are so small, they will pass through the finest filters. They cannot be grown in sugar solutions, but will grow and multiply in the presence of living tissue. They are parasites and depend completely upon their host. Viruses are too small to be seen by ordinary microscopes - they have to be photographed through electron microscope.
Viruses cause many diseases with which we are familiar. In attacking organs of the body, each group of viruses causes a different group of diseases. Some of the diseases caused by virus that attack the skin are chicken pox, smallpox, measles, German measles and fever blisters.
Other viruses cause diseases of nerve tissue, such as rabies, brain fever and infantile paralysis. A third group of viruses cause diseases in the internal organs. Yellow fever, influenza, the common cold and viral liver inflammation are examples of this group.
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