Viruses:

Viruses are micro-organism closely related  to bacteria. These living and self- propagating bodies are not visible even with the strongest compound microscope. They are known to be made up of particular matter and are filtered out by very fine collodion filters. They cannot be filtered by unglazed porcelain by which bacteria is removed from liquids. Some of the bacteria have been found to exist in virus forms at some stages in their life history. Viruses differ from bacteria in the fact that they are much smaller and they multiply only within suitable host cells (in which they produce changes), giving rise to a range of diseases.