Meningococcal disease is caused by Neisseria meningitidis, a bacterium with human carriers as the only reservoir. It is carried in the nose, where it can remain for long periods without producing symptoms.
Following exposure (inhalation of infective droplets) the carrier state may develop and last for some time. Due to a series of factors, a very low proportion of carriers (less than 1%) will eventually become ill. This most frequently occurs in young children, but a second disease peak is observed among adolescents and young adults.
The clinical picture is very serious, and may present as meningitis and severe blood infection. Timely, appropriate antibiotic therapy can usually cure the meningitis (although serious complications including deafness, neurological problems and even amputations are still possible), whereas the severe blood infection leads to death in about 8% of cases.
Vaccines are available against serogroups A, B, C, Y and W. Most instances of the disease in Europe are caused by serogroups B and C.