H. influenzae, an exclusively human pathogen, is spread by airborne droplets or through direct contact with secretions or fomites.

• Type b (Hib) strains are most important clinically, causing systemic invasive disease, primarily in infants and children <6 years of age.
• Widespread use of Hib conjugate vaccine in industrialized countries has dramatically decreased rates of Hib colonization and invasive disease, but the majority of children worldwide remain unimmunized.
• Both typable and nontypable strains can asymptomatically colonize the nasopharynx.