The COVID-19 pandemic represents a wide-ranging form of involvement from asymptomatic through mild respiratory form to bilateral bronchopneumonia with acute respiratory and multiorgan fatal failure. Patients with comorbidities (obesity, cardiovascular diseases, diabetes mellitus) are particularly at risk of a more severe course of infection. We present a 33-year old lean patient with a medical history of ulcerative colitis on immunosuppressive treatment with Azathioprine, after unsuccessful in vitro fertilization one week before the onset of symptoms, admitted to hospital for two-week-long cough with sore throat with fever ap to 40°C. CT confirmed bilateral bronchopneumonia without etiological detection of the infectious agent. Three PCR tests (two of nasopharyngeal swabs and one of bronchoalveolar lavage (BAL)) were negative for COVID-19, including antigen and antibody tests. Complex parenteral ATB treatment with high-flow nasal oxygen therapy was ineffective, and artificial lung ventilation was indicated for acute respiratory failure. After 4 days antifungal treatment of Fluconazole, condition of patient progressed to hepatic and multiorgan failure and the patient died on day 14 of hospitalization. Post-mortem histological examination revealed the presence of coronavirus in the cells of lung parenchyma. The case recalls that even young patients with immunosuppressive treatment are at risk for the critical course of COVID-19 disease. The negativity of the tests was due to the capture of the patient only after the second week of infection, at the time of the diagnostic window between the positive PCR test and the formation of antibodies. The persistent effect of immunosuppression was most likely the reason for the lack of antibody response.