A 33-year-old patient presented to their physician’s office…

A 33-year-old patient presented to their physician’s office with several days of fever and mild headache. A viral illness was suspected, and no treatment was given. Four days later, they returned to the office with low-grade fever, mild cough, and a single episode of blood-tinged sputum. At this time additional history of the patient was obtained. The patient, a mechanic in the US Army, had spent 1 week at a military base in southern California and had returned 10 days before the initial office visit. The most likely agent causing the disease is: