Zak is a 3 year old boy who is brought in by his mum who is concerned about an itchy rash over his face and trunk which she first noticed last night. His nursery commented that he was off his food yesterday and that he had a fever of 37.8℃ prior to collection. Otherwise he has no other symptoms, has had no further fever and appears happy in himself today. He is up to date with immunisations, has no other past medical history other than well controlled Eczema and is on no regular medications. He lives at home with his parents and older sister, none of whom are unwell or have a rash.
On examination there are erythematous papules seen on his face, back and stomach and one vesicle on the back. There are no oral lesions and he is apyrexial.
What is his most likely diagnosis?