The bacteria only rarely give rise to meningococcal disease, but when they do infection spreads rapidly and is fatal in about 10% of cases. If disease is diagnosed early and treated promptly most people make a full recovery.
About 1 in 8 people who recover experience some long term effects. These can include headaches, stiffness in the joints, epileptic fits, deafness and learning difficulties.
More top news
Vaccination programme "temporarily suspended" after several children reported dead, but World Health Organisation says sabotage "unlikely".
Nothing could distract Japanese flutist Yukie Ota from her performance - not even a butterfly which decided to land on her face.
Alexander Hennessy captured the moment an armed robber pointed a gun at him as he cycled through Buenos Aires on his helmet-mounted GoPro.