immunodeficiency

n. inability of the immune system to fight off infections due to a genetic or acquired disease

In medicine, immunodeficiency (or immune deficiency) is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Most cases of immunodeficiency are either congenital or acquired. A person who has an immunodeficiency is said to be immunocompromised. An immunocompromised person is very vulnerable to opportunistic infections. Boys more often than girls can have immune deficiency. Mostly immune deficiency is recognized in the early ages unless it is an acquired immune deficiency.
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Immunodeficiency disorders are a group of disorders in which part of the immune system is missing or defective. [more]Immunodeficiency - Community and Resources

Noun
1. immunological disorder in which some part of the body's immune system is inadequate and resistance to infectious diseases is reduced
(hypernym) immunological disorder
(hyponym) AIDS, acquired immune deficiency syndrome

Inabillity to mount a normal immune response. Immunodeficiency can be due to a genetic disease or acquired as in AIDS due to HIV.