A 30-year-old male developed chills, high fever (39.1°C), dry cough, and right-sided chest pain worsened by deep breathing or coughing after a tiring business trip. Physical examination shows an acutely ill appearance, facial flushing, herpes labialis, and bronchial breath sounds in the right mid-to-lower lung. The clinical diagnosis is acute pneumonia. What is the most likely pathogen?