A one-and-a-half-month-old girl presented with fever (38°C) of 48 hours' evolution and irritability, associated nasal mucus; on examination she had a small erythematous rash and the rest of the examination was normal.
Initial CBC: leukocytes 6310/µl (67% segmented, 6% lancets, 19% lymphocytes, 8% monocytes), haemoglobin (Hb) 10.9 g/dl, haematocrit (Hto) 29.6%, platelets 492 000/µl, C-reactive protein (CRP) 8.9 mg/l. Lumbar puncture: glucose 51.3 mg/dl, protein 133.9 mg/dl (haemorrhagic fluid).
Given the suspicion of severe bacterial infection, she was admitted for intravenous antibiotic treatment with cefotaxime and ampicillin (200 mg/kg/day), adding acyclovir (60 mg/kg/day) in the following 24 hours to cover the herpes simplex virus. Febrile after 48 hours. On the fourth day he presented irritability, poor colour and general condition, with persistent evanescent rash, and respiratory distress; a control blood test was performed with the following findings: leukocytes 2670/µl (segmented 31%, keys 3%, lymphocytes 47%, atypical lymphocytes 5%, monocytes 11%), reticulocytes 0.66%, Hb 8.0 g/dl, Ht 25.3%, platelets 131 000/µl. Capillary blood gases: pH 7.32, PCO2 43.3 and HCO3 21.8 mmol/l. Chest X-ray: bilateral alveolar interstitial infiltrates, more striking on the left side, suggestive of acute pulmonary oedema. Given the finding of pancytopenia, samples were taken for parvovirus and enterovirus serology, as well as influenza A serology. High-flow oxygen therapy was started and he was transferred to the paediatric intensive care unit of a tertiary hospital, where he remained for five days.

Immunoglobluin M (IgM) antibodies for parvovirus B19 were positive and IgG negative, leading to a diagnosis of parvovirus B19 infection, respiratory failure and associated pancytopenia. The patient showed progressive improvement on the fifth day of admission, and was discharged on the twelfth day without further complications. The girl's brother subsequently presented with fifth-day erythema infectiosum.