Neisseria meningitidis , the bacterium that causes meningococcal meningitis in humans, the only natural hosts in which it causes disease.
Meningococci are spherical, frequently occur in pairs, and are strongly gram-negative (see gram stain ). They enter the nasal passage and may cause no symptoms (up to 30% of the population may harbour them between epidemics), or they may enter the bloodstream and produce the symptoms of meningitis.