Haemophilus influenzae type B (Hib) is a type of bacteria, not to be confused with the disease influenza, a lung infection caused by a virus.

Prior to the availability of the Hib immunization, H. influenzae was the leading cause of bacterial meningitis in children under 5 years of age. It occurs most frequently in children from 1 month up to 4 years with a peak at 6 to 9 months.

Routine Hib immunization has resulted in a marked decrease in the incidence of H. influenzae meningitis. H. influenzae meningitis may follow an upper respiratory infection and may develop slowly or rapidly. The infection usually spreads from somewhere in the respiratory tract to the bloodstream and then to the meninges (the membranes that cover the brain). At the meninges, the bacteria produce infection and inflammation causing serious illness and sometimes death.

Since the introduction of the vaccine, H. influenzae now occurs in less than 2 in 100,000 children.

Risk factors include a recent history of otitis media (ear infection), sinusitis (infection of sinuses), pharyngitis (sore throat), or other upper respiratory infection or a history of a family members with an H. influenzae infection. Another significant risk factor includes race -- Native Americans and Eskimos have a rate of more than 3 times that of the general population. Placement in day care also increases risk.