Meningococcal disease refers to any illness caused by bacteria called Neisseria meningitidis, also known as meningococcus [muh-ning-goh-KOK-us]. These illnesses are often severe and can be deadly. They include infections of the lining of the brain and spinal cord (meningitis) and bloodstream infections (bacteremia or septicemia).

This is called meningism or pseudomeningitis. Meningitis caused by the bacterium Neisseria meningitidis (known as "meningococcal meningitis") can be differentiated from meningitis with other causes by a rapidly spreading petechial rash, which may precede other symptoms.

The term meningitis is often applied to meningococcal meningitis, which is caused by Neisseria meningitidis, known commonly as meningococcus. Meningococcal meningitis is worldwide in distribution. It is primarily a disease of youth and especially of children under age 10, though all ages may be affected.