A previously well 16-year-old girl was admitted for fever for 2 days with progressive ‘weakness’. She had coryzal symptoms for 2–3 weeks before admission and started to have a high fever 2 days ago. She felt nauseous and vomited undigested food several times, but there was no diarrhoea. She had poor appetite and felt weak. There was no urinary frequency or dysuria. Her menstrual period was normal, and she denied any sexual activity. Given the vagueness and nonspecific nature of the symptoms, what might explain them?