Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that affect the patient's immune system. Examples of these extrinsic factors include HIV infection and environmental factors, such as nutrition.[1] Immunocompromisation may also be due to genetic diseases/flaws such as SCID.

Reference
Wiki: Inherited immunodeficiency diseases