Meningococcal infection, first recognized over 2 centuries ago as epidemic cerebrospinal fever, occurs worldwide as endemic sporadic cases but with the potential to spread and expand into an epidemic. Humans are the only natural host for the bacteria. Transmission of the organism occurs from person to person by direct contact with colonized respiratory secretions or airborne droplets with subsequent colonization of the nasopharynx. Nasopharyngeal carriage approximates 5% to 15% in non-epidemic periods but may approach 50% to 95% during epidemics. The carriage rate is also increased when there is crowding, such as in military barracks, dormitories, prisons, convocations, and sporting events. The oropharyngeal and nasopharyngeal carriage may persist for several weeks to several months as part of the normal nasopharyngeal flora. Sexual transmission of meningococci in women and homosexual men may result in anogenital carriage.

Most cases of disease (e.g., bacteremia, meningitis) occur in children between 6 months and 5 years of age (Figure 143.1). However, case-fatality rates are highest in the 15- to 24-year age group. With rare exceptions, invasive meningococci have a polysaccharide capsule that forms the basis for serogrouping of strains, and, except for serogroup B, is the principal bacterial antigen to which protective immunity develops (see below). Invasive disease occurs almost exclusively in persons who lack specific bactericidal anti-meningococcal antibody to the invading meningococcal strain.