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).