A 13,5-year-old girl complains of gingival painfullness and hemorrhage during tooth brushing and eating, halitosis. She has been ill with angina for a week. Objectively: mucous membrane of gums in the area of frontal teeth of her upper and lower jaws is edematic, hyperemic. Apices of gingival papillae are necrotic, they also bleed when touched. There is a thick layer of soft tooth plaque. What is the causative agent of this disease?