Meningitis is most often caused by infection with several species of bacteria, viruses, rarely by fungi and parasites. Bacterial meningitis is the most common and dangerous type of meningitis and can be fatal within 24 hours if untreated. The highest global burden is seen with bacterial meningitis.Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis are the most common causes of bacterial meningitis.

N. meningitidis is one of the most common causes of bacterial meningitis in the world and the only bacterium capable of generating large epidemics of meningitis. Meningococcal meningitis and septicaemia are caused by various serogroups of Neisseria meningitidis (meningococcus) which is an aerobic Gram-negative encapsulated bacterium. At least 12 serotypes of N. meningitidis have been characterized by differences in the polysaccharide capsule, 6 of which (A, B, C, W, X and Y) can cause epidemics. Recent outbreaks of group Y and W135 strains suggest that these serotypes are gaining in importance. Explosive epidemics with incidence rates of up to 1000 cases per 100,000 inhabitants have been reported, particularly in sub-Saharan Africa.

Meningococcal meningitis can affect anyone of any age, but mainly affects babies, preschool children and young people. The disease can occur in a range of situations from sporadic cases, small clusters to large epidemics throughout the world, with seasonal variations. Geographic distribution and epidemic potential differ according to serogroup. The largest burden of meningococcal meningitis occurs in the meningitis belt, an area of sub-Saharan Africa, which stretches from Senegal in the west to Ethiopia in the east.

N. meningitidis can cause a variety of diseases. Invasive meningococcal disease (IMD) refers to the range of invasive diseases caused by N. meningitidis, including septicaemia, arthritis and meningitis.

Population At Risk

Although meningitis affects all ages, young children are most at risk. New-born babies are at most risk from Group B streptococcus, young children are at higher risk from meningococcus, pneumococcus and Haemophilus influenzae. Adolescents and young adults are at particular risk of meningococcal disease while the elderly are at particular risk of pneumococcal disease.

People all over the world are at risk of meningitis. The highest burden of disease is seen in a region of sub-Saharan Africa, known as the African Meningitis Belt, especially recognised to be at high risk of epidemics of meningococcal meningitis.

Higher risk is seen when people are living in close proximity, for example at mass gatherings, in refugee camps, in overcrowded households or in student, military and other occupational settings. Immune deficiencies such as HIV infection or complement deficiency, immunosuppression, and active or passive smoking can also raise the risk of different types of meningitis.