A 49-year-old man presents to the clinic because of a sore throat, swollen glands, achy joints, headache, diarrhea, and fever for 3 weeks. The patient reports no chronic medical problems, no travel, and no recent sick exposures. He says he has had unprotected sex with several sex workers (both men and women) in the past year. Vital signs are temperature 38.0°C (100.4°F), blood pressure 121/72 mm Hg, heart rate 82/min, and respiratory rate 16/min. Physical examination is significant only for pharyngeal erythema without exudate and cervical lymphadenopathy. The genital exam is unremarkable. What disease is most likely causing this patient’s symptoms?