A 6-year-old girl has a blotchy, reddish-brown rash on her face, trunk, and proximal extremities that developed over the course of 3 days. On physical examination, she has 0.2-cm to 0.5-cm ulcerated lesions on the oral cavity mucosa and generalized tender lymphadenopathy. A cough with minimal sputum production becomes progressively worse over the next 3 days. Which of the following viruses is most likely to produce these findings?