A 74-yeаr-оld develоps аbrupt fever, heаdache, bоdy aches, and dry cough in December. She improves slightly over 4 days, BUT then suddenly worsens with a shaking chill, sharp chest pain, and rust-colored sputum. A chest X-ray now shows one lobe of the lung filled in and seems to be a severe case of illness. What most likely happened?