Meningococcus is another term for the bacterial species Neisseria meningitidis, which causes the type of meningitis which usually underlies this syndrome. Meningococcal meningitis occurs most commonly in children and young adults, and can occur in epidemics. In the United States it is the cause of about 20% of meningitis cases. At one time it was common among military recruits, but administration of the preventive meningococcal vaccine has greatly reduced this number. Freshman college students living in dormitory housing who have not been vaccinated are another risk group.