Invasive meningococcal disease (IMD) is a serious illness that can lead to disabling functional sequelae and/or be life-threatening, with a high case-fatality rate (estimated at approximately 10% of cases in France). It is caused by a bacterium, Neisseria meningitidis, which is found exclusively in humans and is a commensal of the nasopharynx. Twelve serogroups have been described, determined by the characteristics of the polysaccharide present on the bacterial capsule. The strains most frequently responsible for IIM belong to 5 serogroups: A, B, C, Y, and W135. Meningococcal transmission is airborne, requiring close and/or direct contact, as nasopharyngeal secretions (Flügge droplets) are infectious. Acquisition of this bacterium and colonization of the nasopharynx are rarely followed by IIM. There are multiple factors contributing to its onset: those related to the bacterium (virulence of the strain) and/or those related to the host (immune deficiency, particularly impairment of the complement pathway, and the condition of the respiratory mucosa, particularly following influenza). IMM manifests in various clinical forms, the most common of which are meningococcal meningitis and septicemia, which can be complicated by purpura fulminans and fatal septic shock. With regard to IMM, the National Reference Center for Meningococci provides microbiological expertise, surveillance, and early warning services.