Description: Read about Meningococcal disease, which refers to two conditions caused by the bacterium Neisseria meningitides. Includes transmission, symptoms and treatment.Meningococcal disease refers to two conditions caused by the bacterium Neisseria meningitides: meningococcal meningitis, an inflammation of the tissues that cover the brain and spinal cord, and meningococcemia, an infection of the blood