Complications after the introduction of the tuberculosis vaccine develop extremely rarely. They become accidentally vaccinated infants with undiagnosed severe immunodeficiency. The article considers a clinical case of the course of disseminated BCG infection with multiple organ damage in congenital immunodeficiency in a 9-month-old boy. The disease manifested itself acutely, the child’s condition worsened within a week, and its outcome was the death of the baby. Primary immunodeficiency was confirmed by the results of a pathoanatomic examination. The discussion provides a classification of complications after the introduction of BCG vaccine, the algorithm of the doctor’s actions in case of a suspected complication after vaccination, and also describes the main signs of immunodeficiency.