Meningococcal disease refers to any illness that is caused by Neisseria meningitidis bacteria. The two most severe and common illnesses caused by these bacteria include infections of the fluid and lining around the brain and spinal cord meningitis and bloodstream infections bacteremia or septicemia. Even if they get treatment, about 10 to 15 out of people with meningococcal disease will die from it. Meningococcal disease can spread from person to person. Teens and young adults are at increased risk for meningococcal disease.