Introduction: Malignant pertussis is a severe and life-threatening form of Bordetella pertussis infection, characterised by profound leucocytosis, pulmonary hypertension, and respiratory failure, particularly in young infants. Early recognition and aggressive management are critical to improve outcomes. We present a case of malignant pertussis with concurrent COVID-19 infection in a 42-day-old infant. Case presentation: A 42-day-old infant presented with a five-day history of worseningrespiratory distress. On admission, the infant demonstrated moderate to severe respiratory distress requiring high-flow nasal oxygen, later escalating to bubble CPAP. Nasopharyngeal testing was positive for both Bordetella pertussis and SARS-CoV-2. Laboratory investigationsrevealed marked leucocytosis (WBC 45 ×10? /L) with lymphocytosis (21 ×10? /L),haemoglobin 150 g/L, thrombocytosis (platelets 607 ×10? /L),elevated C-reactive protein (30 mg/L), lactate 3 mmol/L, and pCO? 50 mmHg. Chest radiography showed evolving pulmonary consolidation. Given significant hyperleukocytosis and clinical deterioration, two exchange transfusions were performed. The infant received a five-day course of azithromycin and required paediatric critical care support. Gradual clinical improvement was observed, and the infant was discharged home after 16 days of hospitalisation. Conclusions: Malignant pertussis remains associated with high morbidity and mortality in young infants. Severe leucocytosis should prompt consideration of exchange transfusion to reduce the risk of cardiopulmonary compromise. This case highlights the importance of early escalation of respiratory support and aggressive management, particularly in the context of concurrent viral infection.