A 14-year-old boy complains of rash on the lips, pain while talking and eating. These presentations showed up three days ago. Similar rash has appeared 1-4 times a year for three years. Objectively: general condition is satisfactory, the body temperature is of 36,9°C. On the vermilion border of the lower lip and the skin below there are multiple small grouped vesicles with serous content, and crusts. What is the etiology of the disease?