A normally healthy 8-year-old boy from Florida is visiting friends on a farm in Iowa during the month of July. He presents on July 28 with a fever, cough, and lower respiratory symptoms (no upper respiratory tract symptoms). He has been ill for 4 days. His chest sounds are consistent with pneumonia, so a chest radiograph is obtained. The radiograph shows small, patchy infiltrates with hilar adenopathy. His blood smear shows small, nondescript yeast forms inside monocytic cells. What is the most likely causative agent?