gram-negative, aerobic bacteria; a commensal and pathogen only of humans, and can be carried asymptomatically in the nasopharynx; when found in cerebrospinal fluid it is the causative agent of cerebrospinal meningitis; also found in venereal discharges and blood; there are at least 13 serogroups based on antigenic differences in the capsular polysaccharides; the ones causing most meningitis infections being A, B, C, Y, and W-135; each serogroup can be further classifed by serotype, serosubtype, and immunotype.