This bacterium is only found in people and never in animals or in the general environment. Most people who harbour these bacteria have no symptoms of disease. It is not entirely understood why some people become ill following carriage of Neisseria meningitidis, while others do not.
Cases can occur sporadically and in epidemics.
Children and young adults are most affected. A, C, Y and W-135, it must be appreciated that vaccines will only abort or control outbreaks caused by the specific serogroups contained in the vaccine. There is currently no licensed vaccine effective against group B meningococci. It must be emphasised that there are vaccines available against some of the other organisms, which cause bacterial meningitis. These vaccines, however, do not protect against infection by the meningococci.