Elisabeth is a first child so her parents have little experience of common childhood illnesses.In addition, her mother is an anaesthetist and may be reluctant to bother a general practitioner unless it is really necessary. Unusually, Elisabeth has had two episodes of a severe infection–periorbital cellulitis–for which investigation for immune deficiency may be indicated. Her parents seek advice after Elisabeth has been unwell for three days with fever, irritability, sore mouth, conjunctivitis, rash, and desquamation and has not passed urine for 24 hours. At …