Haemophilus influenzae is a leading cause of bacterial menigitis, primarily in young children and is a significant cause of pneumonia in the elderly and individuals with chronic lung disease. Humans are the only natural host for H.influenzae, which is a normal component of the upper respiratory tract flora.

H.influenzae may produce a capsule of which six distinct types have been identified, or may remain unencapsulated. Upon transmission through inhalation of respiratory droplets, an H.influenzae infection can progress locally within the respiratory tract causing otitis media, sinusitis, epiglottitis and bronchopneumonia or can invade the bloodstream and spread to other parts of the body causing the more serious disorders such as menigitis, septic arthtitis cand celulitis. The local infections are largely associated with unencapsulated H.influenzae whereas encapsulated b strains are commonly associated with the more severe, invasive forms of disease.

Treatment. The severe, invasive forms of the disease are treated with a suitable antibiotic such as ceftriaxone or cefotaxime. The less severe diseases can be treated with penicillins, however with the emergance of β-lactamase-mediated ampicillin resistance, combination therapy can be used.

Vaccines. The current vaccine consists of a Hib capsular carbohydrate conjugated to a carrier protein. The vaccine is generaly administered to children under the age of two and is effective in preventing invasive disease and also in lowering the level of Hib carrioage in the respiratory tract.