A 45 year old woman presents to her physician because of a severe "sore throat." Physical examination demonstrates fever and an extremely tender, enlarged thyroid gland, but no throat erythema. Serum thyroid studies demonstrate a mild degree of hypehyroidism. Two months later, the patient is asymptomatic, and thyroid function tests have returned to normal. She never again experiences difficulty with her thyroid function. Which of the following was the most likely cause of her hypehyroidism?