HiB is one of many strains of the Haemophilus influenzae bacteria and is identified by a particular type of sugar on its cell wall. HiB frequently resides in the mouth and nose without causing a problem. This is called the asymptomatic carrier state and can occur from only 4 percent of the time in the community to over 40 percent of the time in hospital day cares. Thus, it is very common to be exposed to HiB in the course of daily activities.
However, occasionally HiB will invade other parts of the body and cause a variety of infections, including meningitis (an infection of the brain and spinal cord), epiglottis (an infection and swelling of the covering of the esophagus, which can lead to blockage of the breathing passages), and pneumonia.