Immunodeficiency or immune deficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. The immune system protects the body from potentially harmful substances called antigens. Immunodeficiency can be traced to the failure of one or more parts of the immune system. Immunodeficiency can be classified into two types: primary immunodeficiency caused by congenital/inherited defects or secondary immunodeficiency caused by a disease that affects the immune system. Most cases of immunodeficiency are secondary but some people are born with defects in their immune system, or primary immunodeficiency. Primary immunodeficiencies are disorders in which part of the body's immune system is missing or does not function properly. Most primary immunodeficiencies are genetic disorders and the majority is diagnosed in children under the age of one, although milder forms may not be recognized until adulthood. Primary Immunodeficiencies are caused by congenital/inherited defects. It can be grouped according to the part of the immune system that is faulty like B cell deficiencies, T cell deficiencies, combined T cell and B cell deficiencies, defective phagocytes, complement deficiencies. They also can occur in the innate immune system. Secondary immunodeficiency, also called Acquired immunodeficiency, is more common than congenital immunodeficiency. It is the result of an infectious process or other disease like human immunodeficiency virus causes acquired immunodeficiency syndrome (AIDS). Malnutrition, particularly with lack of protein, and many cancers, can also cause immunodeficiency. It may also be caused by chemotherapy which often reduces the number of white blood cells available to fight infection. Examples of types of infections that can lead to immunodeficiency are chickenpox, cytomegalovirus, German measles, measles, tuberculosis, infectious mononucleosis (Epstein-Barr virus), chronic hepatitis, lupus, and bacterial and fungal infections.