Tuberculosis (TB) is an important opportunistic infection in HIV-positive people. We are reporting a case of a 31-year-old HIV-infected patient who was hospitalized in July 2021 for dyspnea, cough with mucopurulent sputum and asthenia. He was confirmed to have Serratia liquefaciens pneumonia and acute respiratory failure. The evolution was unfavorable despite the antibiotic, pathogenic and symptomatic treatment. Because the patient had severe immunosuppression (CD4 count = 37 cell/mm3), we used QuantiFERON-TB Gold Plus for the detection of the Mycobacterium tuberculosis infection. The antituberculosis therapy was initiated, which resulted in a significant improvement of the general condition and the patient was discharged with the recommendation to continue antiretroviral therapy, antituberculosis treatment and Trimethoprim/Sulfamethoxazole—single tablet daily for the prophylaxis of Pneumocystis pneumonia.