pre – calcitonin level in all groups of patients was useful in differential diagnosis of meningitis because mean pre – calcitonin concentration was 61mg (4.8-33.5) in bacterial meningitis and 0.33mg (0-1.7), in viral, after antibiotic therapy, pre-calcitonin level decreased to low level than cannot be detected so pre – calcitonin level is sensitive and specific mediator for differential diagnosis of viral and bacterial meningitis. The gene discussed is CALCA; the disease is meningitis.