Neisseria meningitidis frequently lives in the upper respiratory tract without causing visible signs of illness. Some event is thought to trigger the onset of aggressive behavior of the bacteria, and sporadic cases of meningococcemia and meningococcal meningitis appear. Family members and those closely exposed to an infected individual are at increased risk. The infection occurs more frequently in winter and early spring. It is transmitted from person to person by respiratory droplets.
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