Key points: - Persistence, with a very low incidence of invasive H. influenzae type b infections in adults and children. - Among the other capsular types, type f is only rarely encountered. - Non-capsulated (untypable) strains play a significant role in adults and the elderly (bacteremias of pulmonary and gynecological-obstetric origin) and in children (bacteremias in the neonatal period). - Antibiotic resistance is particularly associated with β-lactams due to β-lactamase production and target modification. This latter resistance mechanism is observed in untypable strains; it confers low-level resistance and does not appear to have had therapeutic or clinical consequences. (R.A.)