any deficiency in the ability to an effective immune response. It may be due to various causes, including aids. 
Immunodeficiency can be caused by contaminants like organochlorine compounds. Even small concentrations of these compounds might weaken the immune responses of an organism, which makes them more vulnerable to bacterial or viral infections.
- Lawrence E (ed.), 2000. Henderson’s Dictionary of Biological Terms. 12th edition. Prentice Hall, Pearson Education Limited. Harlow, Great Britain.