Viruses are ultramicroscopic, acellular, infectious, nucleoproteinous, obligate parasites which can cross the bacterial filter. The extracellular forms of viruses are inert particles and are known as virions. Sometimes the terms 'viruses' and 'virions' are interchangeably used. On the basis of the host cell they infect, viruses are called as zoophages (animal viruses) phytophages (plant viruses), phycophages (algal viruses), mycophages (fungal [...]
Viroids: Viroids are much smaller than viruses and are also considerably simpler, for they consist of no more than a single strand of RNA. The RNA is not enclosed in any structure, and except during infection is not associated with any other chemical substances. The typical viroid is an RNA molecule about 50 nm in length. Though at the present [...]
In this article, we are concerned with the organization and activities of viruses, but it should be emphasized at the outset that viruses are not cells and that it is a moot question whether viruses constitute living systems. Viruses are described here because of their intimate association with cells and because of their contributions to our understanding of certain cellular [...]