MENINGOCOCCUS

the bacterium Neisseria meningitidis, which causes meningococcal meningitis in humans, who are the only natural hosts in which it causes disease. The bacteria are spherical, ranging in diameter from 0.6 to 1.0 mm (micrometre; 1 mm = 10-6 metre); they frequently occur in pairs, with adjacent sides flattened. They are strongly gram-negative. These bacteria enter the nasopharynx; meningococcal infections in the nasopharynx may cause no symptomsup to 30 percent of the population may harbour them between epidemicsor may enter the bloodstream and produce the symptoms of meningitis.