ABSTRACT

Viruses (singular, virus, meaning toxin or poison in Latin) are noncellular, submicroscopic infectious agents that can only replicate inside the cells of another organism. Measuring from 20 to 400 nm (or 10-8-10-6 mm) in diameter, viruses are 10-100 times smaller than prokaryotes (10-7-10-4 mm), 1000 times smaller than eukaryotes (10-5-10-3 mm). Since the majority of the viruses (including those described in the early reports) are small enough to pass through conventional sterilizing lters (0.2 μm), viruses were initially described as lterable agents. Morphologically, viral particles (or virions) vary from simple helical and icosahedral forms, to more complex structures with tails or an envelope. The envelope is composed of lipids and proteins, which may display as spikes in some viruses giving distinct appearance. A major role of the envelope is to protect a virus from adverse external conditions. Underneath lies at least one protein surrounded by a protein shell (known as capsid). The protein capsid guards the nucleic acid within [either a single-or double-stranded nucleic acid made up of ribonucleic acid (RNA) or deoxyribonucleic acid (DNA)] while other proteins (enzymes) enable the virus to enter its appropriate host cells, to reproduce by taking advantage of host cellular machinery, and to evolve within infected cells by natural selection [1].