A 29-year-old plumber presents to the emergency department with a 1-week history of non-productive cough, muscle aches, fever, vomiting and diarrhoea. His observations include temperature 38.4°C, pulse rate 105 bpm, blood pressure 110/76 mmHg and respiratory rate 22/min. On examination, his chest is clear to both auscultation and percussion. A chest X-ray shows bilateral lung basal infiltrates. What is the most likely causative organism ?