“Rob, I know you're a doctor—can I have a word about your son.” The reception class nursery nurse's words chilled me to the bone, as Jamie (aged 5) had been off colour for a few days. “I am a bit worried about the rash he has got on his face. Three other children in the class have a similar rash—do you think it is infectious?”

I looked at the bright red rash on his cheeks and, despite struggling for some time, no diagnosis came …