An 18-year-old patient complains of gingival painfulness and hemorrhage, halitosis, temperature rise up to 38,6°C, general weakness, appetite loss. Objectively: mucous membrane of oral cavity is hyperemic and dry; tongue is covered with white fur, gingival papillae are edematic, their apices have areas of necrotic deposit that can be easily removed leaving bleeding surface beneath. Submaxillary lymph nodes are enlarged, palpatory painful. What is the causative agent of this disease?