Common variable immunodeficiency (CVID): a group of conditions that result in decreased function of the immune system. The affected person is unusually susceptible to infections by bacteria and sometimes viruses. The cause of CVID is unknown in around 90 percent of cases, but it is believed to result from both environmental and genetic (inherited) factors. The genetic changes in CVID are believed to be mutations in genes that control the development and function of B cells, a type of cell involved in the immune response. CVID affects one out of every 25,000 to 50,000 people. Symptoms can begin at any age but most commonly develop in people in their 20s and 30s. In addition to infections, symptoms can include diarrhea and weight loss, due to inflammation of the gastrointestinal tract. Abnormal accumulation of immune cells may cause enlargement of lymph nodes (lymphadenopathy) or an enlarged spleen (splenomegaly). Around 25% of people with CVID also have an autoimmune disorder.