Viruses are sub-microscopic, infectious, nucleoprotein particles that can infect all living organisms. A virus consists of genetic material either in the form of RNA or DNA surrounded by a protein coat.Most of the viruses, infecting plants, have single-stranded RNA as genetic material. On the other hand, the viruses infecting animals have single or double-stranded RNA or double-stranded DNA.Bacteriophages or viruses infecting bacteria mostly have double- stranded DNA. Their protein coat called as capsid is made up of capsomere subunits. These capsomeres are arranged in helical or polyhedral geometric forms.AIDS, smallpox, mumps, and influenza are some common examples of viral diseases.