Invasive meningococcal disease (IMD) refers to serious, rapidly developing infections caused by Neisseria meningitidis, most commonly presenting as meningitis (inflammation of the membranes surrounding the brain and spinal cord) or meningococcemia (septicemia caused by meningococci), and more rarely as septic arthritis or pericarditis. The most severe form is purpura fulminans. Meningococci have humans as their exclusive reservoir, with frequent carriage of the bacterium (estimated at 5–10% of the general population) but very rarely leading to an invasive form. This bacterium possesses a polysaccharide capsule that determines its serogroup. Twelve serogroups exist worldwide. In France, serogroups B and C are most common, with serogroups W135 and Y being less common. The incubation period ranges from 2 to 10 days, with an average of 7 days.