An 18-year-old patient complains about body temperature rise, weakness, pain induced by eating and deglutition. Objectively: mucous membrane of the oral cavity is erythematic with multiple petechia. Pharynx is hyperaemic. Regional lymph nodes are enlarged, mobile, painless. In blood: leukocytosis, monocytosis, atypic mononuclear cells, ESR is 30 mm/h. What is the leading factor of disease development?