Viruses and diseases are biological entities that, although not exclusively malignant, have the ability to impair the health and functioning of a living organism. Specifically, viruses are ultramicroscopic infectious agents, many of which are pathogenic, that reproduce within the cells of living hosts. They are inert in their independent state, but will become reactive when in contact with an organism. Because of this distinct trait, they are characterized as being both living and non-living structures. (Source: WordNet, UCMP/Berkeley / Photo: Flickr)