Bacteria and viruses are the two main causes of meningitis. The bacterium Neisseria meningitidis, also called meningococcus, causes meningococcal meningitis. In children and teens, meningococcus is the most common cause of bacterial meningitis. In adults, it is the second most common cause. Meningococcal bacteria may cause infection in a part of the body -- the skin, gastrointestinal tract, or respiratory tract, for instance.