Patients with etiologically unidentified aseptic meningitis will be treated with symptomatic therapy and will be asked to complete a questionnaire.

Patient with aseptic meningitis in whom serological diagnosis of tick-borne encephalitis will be established. Patients will be treated with symptomatic therapy and will be asked to complete a questionnaire.

Patients with acute aseptic meningitis in whom Lyme neuroborreliosis will be proven or suspected according to microbiological criteria. Patients will be treated with antibiotic therapy (ceftriaxone or doxycycline) and will be asked to complete a questionnaire.

Patients will be asked to refer a spouse to serve as a control . If unmarried they will be asked to refer a family member or a friend of +/- 5 years to serve as a control.

patients will receive symptomatic therapy with antipyretics, analgetics, antiemetics and parenteral hydration: metamizole, paracetamol, thiethylperazine, saline

Selected clinical parameters (presence or absence of headache, nausea, vomiting, neurologic symptoms, maximal temperature, etc) in aseptic meningitis/meningoencephalitis patients at enrollment (at admission to hospital) and at follow-up visits: on day 7 or on day of discharge from hospital, on day 14, at 2, 6, and 12 months.

Selected laboratory parameters (complete blood count, biochemistry test results) in aseptic meningitis/meningoencephalitis patients at enrollment (at admission to hospital) and at follow-up visits: on day 7 or on day of discharge from hospital, on day 14, at 2, 6, and 12 months.

Selected microbiologic parameters (anti Borrelia burgdorferi immunoglobulin M and G in serum and cerebrospinal fluid results, polymerase chain reaction for enteroviruses, herpes simplex virus type 1 and 2 in cerebrospinal fluid results) in aseptic meningitis/meningoencephalitis patients at enrollment (at admission to hospital) and anti Borrelia burgdorferi immunoglobulin M and G in serum at follow-up visits: at 2, 6, and 12 months.