Various dise­a­ses of the immu­ne sys­tem which are cha­rac­te­ri­zed by a tem­pora­ri­ly or irrever­si­b­ly wea­ke­ned immu­ne sys­tem, i.e. abi­li­ty to defend the body against inva­ding patho­gens, are refer­red to as immun­ode­fi­ci­en­cy. Immun­ode­fi­ci­en­ci­es can be con­ge­ni­tal or deve­lop during the cour­se of ano­t­her ill­ness (e.g. leukemia).