A 3-year-old boy is brought to the physician because of a 2-day history of fever and an itchy rash. The rash began on his face and then spread to his trunk, arms, and legs. Several children at his day-care center have had similar symptoms. His temperature is 38.9°C (102°F), pulse is 100/min, respirations are 20/min, and blood pressure is 110/60 mm Hg. Physical examination shows multiple red papules and vesicles over the face, trunk, and upper and lower extremities. Some vesicles contain clear fluid, whereas others are crusted. Which of the following is the most likely causal virus?