A 23-year-old man undergoing chemotherapy for acute lymphoblastic leukemia has developed a fever and abdominal pain within the past week. He now has a severe cough. On physical examination, his temperature is 38.4deg C. On auscultation, crackles are heard over all lung fields. Laboratory studies show hemoglobin, 12.8 g/dL; hematocrit, 39%; MCV, 90 mm3; platelet count, 221,000/mm3; and WBC count, 16,475/mm3 with 51% segmented neutrophils, 5% bands, 18% lymphocytes, 8% monocytes, and 18% eosinophils. Infection with which of the following organisms is most likely to be complicating the course of this patient's disease?