A 19-year-old man developed a severe upper respiratory infection. His chest X-ray was normal and the infection was presumed to have been viral. One week after his respiratory symptoms had resolved, he developed numbness of his entire left side. He reported no weakness, headache, changes in vision, seizures or right-sided symptoms during this period. His MR scan of brain showed an isolated symmetrical, non-expansile lesion in the splenium of the corpus callosum, characterised by diffusion restriction (figure 1A, B) and fluid attenuated inversion …