A 5-year-old boy is brought to the GP by his mother with a 2-day history of fever, tiredness, and a widespread rash. The rash began on his trunk and face as small red spots, but has since progressed to form itchy lesions. His mother reports that the rash seems to be spreading, with some areas already forming crusts. He has not received the varicella vaccine, and several children at his school have recently been absent with a similar illness. On examination, he is mildly pyrexial and appears irritable.