This case study describes the complex diagnostic and therapeutic process in an immunocompetent patient suffering from a disseminated nontuberculous mycobacteria infection caused by Mycobacterium avium complex. During the infection, the patient suffered from pancytopenia, liver damage and acute renal failure. Necrotizing granulomatosis was found in bone marrow and splenic tissue with no signs of infection macroscopically or using imaging methods. Diagnosis was eventually determined by microbial cultivation of splenic tissue. The patient was then prescribed targeted antituberculotic treatment, which led to clinical and laboratory improvement.