This micrograph depicts the presence of aerobic Gram-negative "Neisseria meningitidis" diplococcal bacteria; Mag. 1150X. Meningococcal disease is an infection caused by a bacterium called N. meningitidis or the meningococcus. The meningococcus lives in the throat of 5-10% of healthy people. Rarely, it can cause serious illness such as meningitis or blood infection.