An otherwise healthy 4-year-old boy was brought to hospital in late December after 3 days of nasal congestion and 2 days of fever, vomiting, malaise, ataxia and aphasia. The patient had a medical history of clubfoot. He had not yet received his immunizations (4–6 yr) or the seasonal influenza vaccine. On arrival, his vital signs were normal, but he had truncal and gait ataxia. The remainder of his neurologic and physical examination was normal. There was no nuchal rigidity, no ankle clonus and the Babinski sign was negative.