Neisseria meningitidis brain infection, computer illustration. N. meningitidis are gram-negative diplococci (spherical bacteria arranged in pairs), transmitted via the respiratory route. In most people they cause either asymptomatic infections or non-severe nasopharingitis, but can also cause severe meningitis and generalised infection (meningococcaemia), particularly in children. Their surface is covered with pilli which take part in attachment of bacteria to cells and surfaces.