A 3-yeаr-оld girl is brоught tо you аfter she is noted to hаve small pink spots on her lower extremities and bleeding from her gums. She had a URI a couple of weeks ago. You notice petechial lesions on lower extremities and purpura on her oropharynx. The mother denies any other bleeding issues. You obtain a complete blood cell count which is normal except for platelet count of 15,000 (normal 150,000-450,000 pl/mL) and you suspect immune thrombocytopenia purpura (ITP). Which of the following may have led to this patient's development of ITP?