These exist in nature tiny little organisms which are much smaller than microbes. They are so small that they cannot be seen through an ordinary microscope and their existence came to light only through research carried out with electronic microscopes which can magnify objects millions of times.

These strange organisms, which are still little-known, were called filterable viruses because they could pass through the very finest of ‘nets’ used by scientists to catch bacteria. They are parasitic, in that they multiply only in living cells. The diseases caused by viruses are known as viruses and they affect people, animals and plants.

It is extremely important to establish if a virus, when found, is pathogenic (causing diseases) or non-pathogenic (harmless).