We present a 6-month-old boy with failure to thrive who was referred to the emergency department by his primary care doctor for leukocytosis and was found to be hypoxic with diffuse infiltrates on chest radiograph. Our patient was admitted and eventually diagnosed with Pneumocystis jirovecii pneumonia secondary to hyper immunoglobulin M syndrome. Even though P jirovecii pneumonia is an uncommon cause of hypoxia in infants, this case illustrates the need for pediatric emergency physicians to be cognizant of this rare but life threatening cause of hypoxia.