A 12-year-old child complains of high body temperature of 39.7°C, enlarged lymph nodes, sore throat, runny nose, and cough. Objectively, acute catarrhal stomatitis is observed in the oral cavity, some of the palatine tonsils have a yellow gray coating that crumbles and can be easily removed. The cervical, occipital, and submandibular lymph nodes are enlarged, mobile, and slightly painful. Laboratory blood testing shows leuko-, lympho-, and monocytosis and atypical mononuclear cells. What is the causative agent of this disease?