The epidemiology of N. meningitidis has been reviewed by Cartwright. Humans are the sole natural host for the meningococcus, which is carried asymptomatically as part of the commensal nasopharyngeal microflora in approximately 10% of the general population. The meningococ-cus may occasionally cause invasive disease by traversing the nasopharyngeal mucosa into the bloodstream, where it multiplies rapidly. Although invasive behavior is poorly understood and unpredictable in the individual, it appears to be related to pathogen characteristics (e.g., serogroup, clonal lineage), host factors (e.g., age, smoking, immu-nological abnormalities, concurrent viral infections), and environmental factors (e.g., overcrowding, local climatic factors). The majority of disease occurs in the under 5 years age groups with a peak incidence in the 6-12 months age group. The United States and Europe experience an endemic rate of disease mainly caused by serogroup B, C, and Y meningococci (one to five cases per 100,000 yearly), punctuated by occasional hyperendemic or geographically localized outbreaks. In China and sub-Saharan Africa, epidemic and pandemic diseases caused by serogroup A, and, recently, serogroup W-135 menin-gococci, are common (up to 500 cases per 100,000) with major epidemics occurring every 5-10 years.
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