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See also page on Immunity
Immunodeficiency can be primary or secondary. Genetic causes which affect multiple aspects of the immune system tend to present early in life with overwhelming, severe infection (eg SCID). More specific defects on the other hand, may present later in life with characteristic infections (eg meningococcal disease in complement deficiency), or indeed with no obvious increase in susceptibility to infection at all (eg IgA deficiency).
Natural immunity may be suppressed with drugs eg cytotoxics, steroids either intentionally, in the treatment of autoimmune disease, or incidentally. Such immunosuppression is seen increasingly as new therapies become available for cancers and autoimmune conditions.