A 5-year-old female presents with a low-grade fever and rash. Physical examination reveals a temperature of 38.3°C (100.9°F) and bright red, erythematous rash on the face that is prominent on the cheeks. A blanching, reticulated rash is also noted on the arms and trunk. The patient is up to date on vaccinations and her parents indicate that the facial rash erupted 3 days ago and the body rash erupted this morning. A complete blood count reveals reticulocytopenia. The most likely etiology is ?