Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Common variable immunodeficiency ?
    ### Response:
    What causes common variable immunodeficiency (CVID)? Common variable immunodeficiency (CVID) is usually sporadic and thought to result from a combination of genetic and environmental factors. In most cases, the exact cause of CVID is unknown. Genetic factors associated with CVID include mutations in genes involved in the development and function of immune system cells (B cells) which help protect against infection.
    