Meningitis is an infection of the membranes that surround the brain and spinal cord. If a lot of inflammation accompanies the infection, materials released by the body's defense system damage the underlying tissues in the central nervous system and the nerves that penetrate the infected membrane. Bacterial meningitis often results in brain damage, deafness, and death, even when promptly treated with antibiotics. It's much more likely to be severely damaging than similar infections caused by viruses, though the latter are sometimes pretty bad. (For instance, the reason we immunize children against mumps, a viral disease, is to protect them from the meningitis that sometimes goes along with it. Before the vaccine was introduced, mumps was the leading cause of deafness in children.) Hemophilus influenza type B, or HIB, and pneumococcus used to be more common causes of meningitis than meningococcus. But they have been so diminished by already existing vaccines that some of the pediatric residents in my teaching hospital have never seen a case of meningitis caused by these germs.