• A virus is a Latin Word Meaning “poison


Three main criteria are used to define a virus

a) Sub-microscopic (20 -50 nanometres) obligate intracellular parasites

b) Their genetic element is either RNA or DNA (not both) that is wrapped into a protein coat (capsid)  and some with lipid envelope enclosing the coat

c) Viruses survive and replicate only in living susceptible cells


  • Definition: Viruses are sub-microscopic obligate intracellular parasites which replicate only in living susceptible cells and possess a central core made up of one nucleic acid, either RNA or DNA never both


  • The complete infective virus particles (i.e. capsid enclosing the nucleic acid) which are INERT CARRIERS of the genome (do not grow) are called VIRIONS


Comparing viruses with other microorganisms


A virus differs from a cell in three fundamental ways:

  1. A virus usually has only a single type of nucleic acid serving as its genetic material.  This can be single or double-stranded DNA or RNA;
  2. Viruses contain no enzymes of energy metabolism, and thus cannot make ATP;
  3. Viruses do not encode sufficient enzymatic machinery to synthesize their component macromolecules, specifically, no protein synthesis machinery.


Viruses are distinguished from other obligate parasites, some of which are even simpler than viruses:

MYCOPLASMA: Small bacterium that grows only in complex medium or attached to eucaryotic cells.

CHLAMYDIA: Obligate intracellular bacterial parasite which depends on eucaryotic cells for energy.

PROTOZOA: Obligate intracellular parasites that replicate within eukaryotic cells.

VIROID: Infectious agents of plants that exist as naked nucleic acid (circular single-stranded (ss) ssRNA).

HEPATITIS DELTA VIRUS (HDV): Viroid-like agent whose replication is dependent upon HBV.