Haemophilus influenzae is a gram-negative bacterium that colonizes the nasopharynx mucosa in humans. It was first described in 1883, after isolating it from the purulence of patient sick for conjunctivitis. Ten years later it was mistakenly considered to be an etiology of influenza, hence the word „influenzae” in the species name. In 1933 it was shown that the flu was caused by viruses rather than bacteria. Most H. influenzae strains are a component of the normal bacterial flora of the human nasopharynx. Haemophilus influenzae occurs in the unencapsulated and the encapsulated form. Six serological types of encapsulated strains were defined and labeled A through F. Serotype B is the most dangerous and causes severe illness. Other unencapsulated and the encapsulated strains can also cause illness (sinusitis, ear, throat or lung), but do not cause serious infection. It is estimated that 60-80% of all children are carriers of one of the H. influenzae strains. In countries where mass vaccination has not been introduced, 3-5% of carriers are found to carry encapsulated strain belonging to serotype B. In regions where vaccination is compulsory, type B from the nasopharynus is rarely isolated (<1% of carriers).
Haemophilus influenzae type B is responsible for over 95% of infections caused by Haemophilus influenzae in children and for 50% of infections in adults. The transmission of the bacteria is mainly on the droplet route, which is why infections are more common in nurseries and kindergardens. Strains of serotype B cause both respiratory tract infections, including acute epiglottitis and invasive infections such as meningitis or septicemia.