Acute Bronchiolitis — Practice

A 12-year-old boy with known sickle cell disease presents with sudden severe pallor, lethargy, and a haemoglobin of 3 g/dL. The reticulocyte count is strikingly low at 0.2%, and there is a preceding febrile illness with a slapped-cheek facial rash in his younger sister. Which organism is most likely responsible for this acute event?