Young boy suffering from a rapidly spreading rash

You are called to the emergency department to evaluate a 2-year old boy with a fever for 5 days and a rapidly spreading rash on his face, arms, and legs for 2 days. The rash was made up of 3-mm to 6-mm diameter elongated vesicles on a red base. He is irritable and not eating well. There was another child at his day care with a similar rash, and his mother is worried that his 8-month old brother will develop the same illness.