Haemophilus influenzae, often referred to as H. flu or "HiB" for convenience, is a family of bacteria all of which can cause diseases in people. (The bacteria does not have anything to do with influenza, but when first identified in 1892 it was thought to cause the flu, hence the name.) There are six types of H. flu known; most H. flu-related disease is caused by type B, or "HiB".
Until a vaccine for HiB was developed, HiB was one of the two most common causes of otitis media, sinus infections, and bronchitis. More important, HiB was also the most common cause of meningitis, and a frequent culprit in cases of pneumonia, septic arthritis (joint infections), cellulitis (infections of soft tissues), and pericarditis (infections of the membrane surrounding the heart). One of the most dangerous results of HiB infection was epiglottitis, an infection of the "flap" at the top of the windpipe that can kill a child by blocking airflow to the lungs.
Before the vaccine was introduced, there were about 20,000 serious cases of HiB infections in the United States every year, most of which were of meningitis. Since the vaccine became required, that number has dropped to about one-sixth to one-eighth of what it was. HiB infections are very rare after age 5 years except in people with immune-system problems. Before we were able to vaccinate against it, 1 out of 200 children had severe HiB infections, while two-thirds of HiB infections happened in children less than 18 months old. In some countries, serious HiB infections have almost disappeared, thanks to the vaccine.
HiB vaccine is given 2-3 times in the first 6 months of life, followed by a single dose at age 12-18 months. (There are three different HiB vaccines available; they are all very effective, but the dosage schedule differs between the types.) There is also a vaccine, COMVAXTM, which combines the HiB and hepatitis B vaccines; the schedule for this vaccine is slightly different because HiB vaccine does not work well if it is given before age 2 months, while hepatitis B can be started earlier.)
For more information on HiB and HiB vaccine, see the CDC Web site.