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Severe Combined Immunodeficiency, which is usually known as SCID, is a Primary Immune Deficiency which affects both the T Cells and the B cells of the immune system. The immune system is incredibly complex, but to explain this in a very simplistic way, B cells fight bacterial infections and T cells fight viruses and fungus.

Now, the most common type of Primary Immune Deficiency (PID) is called common variable immune deficiency (CVID) and this affects the B cells of people, making it very hard or impossible for them to fight bacteria. Without treatment people living with CVID will get very ill and may eventually die.