On the 2nd day of disease a 27-year-old patient complains of unbearable headache, repeated vomiting. Objectively: the patient is in a grave condition. He is conscious but adynamic. Lies in a forced position with his head thrown back. There is no skin rash. Nuchal muscles are evidently rigid, there are Kernig's and Brudzinski's signs. t°- 39,5°C, Ps- 120/min, AP- 130/80 mm Hg. The leading syndrome of this disease is caused by: