Overview: epidemiology, surveillance, and population biology.

A combination of data obtained by classical epidemiological techniques with insights gained from the analysis of the population biology of Neisseria meningitidis have proved to be critical in understanding the spread of menin-gococcal disease. This is a consequence of the natural history and evolution of this bacterium, which, despite its fearsome reputation as an aggressive pathogen (1), is ordinarily a harmless commensal inhabitant of the nasopharynx of adult humans (2). Further, it has been established that "natural," (that is to say, carried) populations of meningococci are highly diverse, with a minority of genotypes (the "hyperinvasive lineages") being responsible for the majority of disease (3). Finally, it is known that distinct hyperinvasive lineages tend to be associated with particular epidemiological manifestations of meningococcal disease (4) and some are especially associated with severe disease (the "hyper-virulent" lineages) (5). These complexities have important implications for public-health interventions, as different disease epidemiologies, caused by genetically diverse meningococci, require distinct approaches to public-health management. For example, the public-health response necessary to combat large-scale meningococcal-disease outbreaks in Africa (6) is different from that required during an institutional disease outbreak in Europe and North America, and prolonged geographical outbreaks in these countries require a different response again (7). In recognition of the importance of the multi-disciplinary approach necessary to establish these insights, this section contains chapters ranging from outbreak management through surveillance and isolate characterization techniques to phylogenetic methods. Together the chapters provide the methodologies necessary for monitoring, understanding, and reacting to the spread of meningococcal disease.