Serogroups
Five major serogroups of N. meningitidis, A, B, C, Y and W-135, cause most human infection. Serogroup A is the leading cause of disease in the "meningitis belt" across sub-Saharan Africa, but now occurs only rarely in the US. Serogroups B, C and Y are major causes of endemic disease in the US; serogroup B causes about one third of cases overall and more than half of cases in infants. In the US during the 1990s, the frequency of local outbreaks of meningococcal infection and the proportion of infection caused by serogroup C increased among 15 to 24 year olds, and these outbreaks were associated with a fatality rate of about 20%.2 Serogroup W-135 is an uncommon cause of meningococcal disease in the US, but has caused outbreaks among pilgrims to Mecca during the Hajj and regional outbreaks throughout the world.