Viruses are non-cellular organisms characterised by an inert crystalline structure outside the living cell. They are obligate parasites that are inert outside their specific host cell. Structurally, a virus is a nucleoprotein consisting of genetic material enclosed within a protein coat called capsid. This capsid is made of small subunits called capsomeres, which are arranged in helical or polyhedral geometric forms to protect the nucleic acid.

Regarding the nature of genetic material, viruses contain genetic material that is infectious and could be either RNA or DNA. Crucially, no virus contains both RNA and DNA. The type of genetic material varies with the host: viruses infecting plants usually have single stranded RNA, whereas viruses infecting animals have either single or double stranded RNA or double stranded DNA. Bacteriophages (viruses infecting bacteria) are usually double stranded DNA viruses.

Viruses cause numerous diseases in animals and plants. Four common viral diseases are mumps, small pox, herpes, and influenza. Other diseases include AIDS in humans and mosaic disease in tobacco plants.