The bacteria Haemophilus influenzae type b (Hib) can cause serious illnesses, including pneumonia (infecting the lungs), epiglottitis (infecting the tissue at the back of the throat), and meningitis (infecting the tissues around the brain and spinal cord). The Haemophilus influenzae type b vaccine has dramatically decreased the cases of serious illness caused by the bacteria.

Most serious Hib disease affects children between 6 months and 1 year of age. Symptoms include a sore throat and fever. Until a few years ago, Haemophilus influenzae type b was the number one cause of meningitis and epiglottitis in young children.