Immunodeficiency is the state of lacking adequate immunity to disease — the immune system’s defences absent, reduced, or malfunctioning. Primary immunodeficiencies are inherited defects (severe combined immunodeficiency, the “bubble boy” disease); secondary immunodeficiency follows HIV infection, chemotherapy, malnutrition, or immunosuppressive drugs. The result is vulnerability to infections — recurrent, severe, or from organisms harmless to others — and sometimes to autoimmunity and cancers. Treatment replaces what is missing: immunoglobulin infusions, bone-marrow transplantation, antiretroviral therapy, or simply feeding the malnourished child whose immune system starves with them. In nutrition’s hardest settings, immunodeficiency and malnutrition form a vicious circle, each worsening the other.