Haemophilus influenzae type b (Hib) is bacteria that commonly cause bacterial meningitis and pneumonia and the leading cause of other invasive diseases as septic arthritis (joint infection), epiglottitis (infection and swelling of the epiglottis) and cellulites (rapidly progressing skin infection which usually involves face, head, or neck).  pericardiatis (infection of the sac covering the heart) and Osteomyelitis (bone infection) are  less common forms of invasive disease.

Otitis media and acute bronchitis due to H. influenzae are generally caused by nontypeable strains. Nontypeable strains are rare causes of serious infection among children but are a common cause of ear infections in children and bronchitis in adults.

Hib diseases mainly affect children under five years. Elderly, unvaccinated or incompletely vaccinated children and people with a weakened immune system are at increased risk of acquiring the Hib infection.

Recurrent infection with Hib is likely among children less than 24 months; however, immunity against the disease is likely to be developed among those become infected with Hib at 24 months of age or older.