Haemophilus influenzae can cause serious invasive disease, especially in young children. Invasive disease is usually caused by encapsulated strains of the organism. Six typeable capsular serotypes (a–f) are known to cause disease; non-typeable encapsulated strains can occasionally cause invasive disease and non-encapsulated strains are mainly associated with respiratory infections such as exacerbation of chronic bronchitis and otitis media. Before the introduction of the conjugate vaccine into infant immunisation programmes, type b (Hib) was the prevalent strain. The most common presentation of invasive Hib disease is meningitis, frequently accompanied by bacteraemia (60% of cases). Fifteen per cent of cases present with epiglottitis and bacteraemia, without any other concomitant infection, occurs in 10% of cases. The remainder is made up of cases of septic arthritis, osteomyelitis, cellulitis, pneumonia and pericarditis. The sequelae following Hib meningitis may include deafness, seizures, and intellectual impairment and the case fatality rate from Hib meningitis is 4–5%.

Individuals can carry Hib bacteria in their nose and throat without showing signs of the disease. Before Hib vaccine was introduced, about four in every 100 pre-school children carried the Hib organism; after the vaccine was introduced, carriage rates fell below the level of detection. Hib is spread through coughing, sneezing or close contact with a carrier or an infected person.