Download Virus - World Health Organization

Virus
Definition
A sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell
(WHO 2009).
A minute organism not visible by light microscopy (Symons et al. 2000). See also : Infectious disease.
Explanation
A virus is an obligate parasite dependent on nutrients inside cells for its metabolic and
reproductive needs. It consist of a strand of either DNA or RNA, but not both, separated by
a protein covering called a capsid (Symons etal. 2000).
Viruses consist of two or three parts : all viruses have genes made from either DNA or RNA
(but not both), long molecules that carry genetic information; all have a protein coat called
a capsid that protects these genes; and some have an envelope of fat that surrounds them
when they are outside a cell. They are about 100 times smaller than bacteria (WHO 2009). Example
A range of disease are caused by infections with viruses, ranging from influenza and
measles to yellow fever, dengue and various forms of encephalitis (WHO 2009).
Reference for Definition
Symons JM, Bradley LC and Cleveland TC (2000) The drinking water dictionary, American
Water Works Association.
WHO(2009) Health impact assessment glossary: E-learning modules. Useful Links
(c) World Health Organisation
www.who.int/thelexicon