Meningococcal disease refers to a variety of clinical signs and symptoms caused by the bacterium Neisseria meningitidis or N. meningitidis, which is commonly referred to as meningococcus. N. meningitidis causes both isolated cases of disease and epidemics of disease. The most common types of meningococcal disease are infections of the blood (meningococcemia) and infections of the coverings of the brain and spinal cord (meningitis). Approximately 3,000 cases of meningococcal infection occur annually in the United States. The incidence of meningococcemia is highest among children from three to twelve months of age and declines among older age groups (figure 1). In some previously healthy persons the disease progresses rapidly, resulting in death within hours of the onset of symptoms. Close contacts of persons with meningococcal disease are at highest risk for acquiring the disease. Persons who have certain medical conditions are at an increased risk for acquiring meningococcal infections, for developing severe infections, and for having repeat infections. Meningococcal disease is more common among persons with defects in certain parts of their immune system (for example persons with terminal complement deficiency (C3, C5-9) and persons without spleens), persons receiving immunosuppressive drugs, or persons with autoimmune diseases such as lupus erythematosus. Infection with the human immunodeficiency virus (HIV) is not a defined risk factor, but cases of meningococcal disease have been reported in HIV infected persons.