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Meningococcal disease occurs as a result of a systemic bacterial infection by Neisseria meningitidis. Meningococci are gram-negative diplococci, divided into antigenically distinct serogroups. They are classified according to characteristics of the polysaccharide capsule into capsular group, and of outer membrane proteins into type and subtype. Further characterisation, undertaken by sequencing several other regions of the chromosomal DNA, defines the sequence type (ST).