Primary Immunodeficiency: are a group of 350 rare diseases, chronic disorders in which part of the body's immune system is either missing or function improperly. While these aren't contagious, these diseases can be caused by hereditary or genetic defects. Some of these diseases affect a single part of the immune system while others may affect one or more components. All diseases differ, but they all share one common feature and that is a defect in one of the body's normal functioning immune system.