Haemophilus influenzae is a gram-negative bacillus that lives symbiotically in the upper respiratory tracts of humans. With the occasional exception of the human genital tract, the human nasopharynx is the sole ecologic niche occupied by this organism; H. influenzae is not found in the environment and does not colonize, or infect, other animal species. Epidemiologic studies have documented that H. influenzae nasopharyngeal colonization rates vary between 25 and 80% in humans sampled (80), with higher rates for children than for adults. In addition to asymptomatically colonizing humans, H. influenzae also causes significant human disease. H. influenzae strains that possess the type b capsule (Hib), composed of polyribose-ribitol phosphate (PRP), cause life-threatening, sometimes fatal, infections such as bacteremia and meningitis in nonimmune infants and children. Vaccines consisting of the PRP capsule conjugated to various protein carriers are highly immunogenic, even in very young children (81) and are protective against serious Hib infections (13). Although widespread use of these vaccines in the United States has resulted in the nearly complete disappearance of serious Hib infections in children, they do not protect against H. influenzae strains that possess one of the other types of capsule (a or c to f) or against those strains that possess no capsule (so-called nontypeable, as they do not react with antisera specific for capsular types a through f). Nonencapsulated (nontypeable) H. influenzae rarely causes life-threatening, invasive infections in otherwise healthy children or adults but is a significant cause of localized respiratory infections, such as otitis media, sinusitis, pneumonia, and bronchitis.

Nasopharyngeal colonization of H. influenzae, which precedes H. influenzae infection and disease, is a dynamic process (79). Colonization begins early in life and may persist for prolonged periods of time (21, 33, 75, 77). Epidemiologic studies, which utilize recently developed molecular typing systems that more accurately define bacterial strain differences, show rapid turnover of strains in some individuals—the duration of colonization by a single strain may be less than 1 month. In addition, chronically colonized individuals acquire variants of their original H. influenzae strain that differ in their surface antigens, and individuals may be cocolonized with more than one H. influenzae strain (33, 75, 77).