Haemophilus influenzae (including Hib) is a bacterium that can cause a severe infection, occurring mostly in infants and children younger than five years of age. It can cause lifelong disability and be deadly. In spite of its name, Haemophilus influenzae bacteria do not cause influenza (the “flu”).
There are six identifiable types of Haemophilus influenzae bacteria (a through f) and other non-identifiable types (called nontypeable). The one most people are familiar with is Haemophilus influenzae type b, or Hib. There’s a vaccine that can prevent disease caused by Hib, but not the other types of Haemophilus influenzae bacteria.