27-year-old white man presents to his family doctor complaining of being tired all the time and having a slight fever for the past two weeks. He recently returned from a trip to Las Vegas, where he indulged in some of the infamous nightlife. His physical exam is unremarkable except for a macular rash over his trunk and on the palms of his hands. There are no lesions or ulcers on the penis. What organism is causing this man's illness?