Meningococcal disease is a rather rare disease caused by the bacterium Neisseria meningitidis. Fortunately, it's popularity is at an all time low in the United States, steadily declining since the 1990s, far before the routine use of its vaccines.
A child with meningococcal disease
Meningococcal disease has five serogroups, or strains: A, B, C, W, and Y. Three of the serogroups (B, C, and Y) cause the most illnesses in the United States, and currently have vaccines to fight against the bacteria. However, like any vaccine, they're not always 100% effective.
Meningococcal is spread by exchanging respiratory and throat secretions during close or lengthy contact, but it is not as contagious as the common cold or flu. About 1 in 10 people have the bacteria in their throat with no signs or symptoms; they are called carriers.
The African Meningitis belt shows infection rates as high as 1 in 1,000 to 1 in 100 before vaccine introduction in 2010
Every year, about 2,500 to 3,500 people become infected in the United States, with a frequency of about 1 in 100,000. Of those who contract the disease, 10-15% will die. Of those who survive, 1 in 5 will have permanent disabilities.
Meningococcal vaccines contain antigens that cause the body to make antibodies, which protect and kill bacteria invading the immune system. The vaccines are not live, and therefore cannot cause the disease itself; they are actually made up of the antigens from the outer polysaccharide capsule that surrounds the bacterium.
The bacteria spreads through the bloodstream to the nervous system, causing meningococcal meningitis.