There are two main categories of diseases that affect the immune system: autoimmune diseases and immunodeficiency diseases. Autoimmune diseases occur when the immune system loses self-tolerance and attacks the body's own tissues, examples include systemic lupus erythematosus, rheumatoid arthritis, and scleroderma. Immunodeficiency diseases weaken the immune system, leaving the body vulnerable to infection; examples are severe combined immunodeficiency, DiGeorge syndrome, agammaglobulinemia, and acquired immunodeficiency syndrome (AIDS) caused by the human immunodeficiency virus (HIV).