Haemophilus influenzae can be demonstrated as a saprophyte in more than two-thirds of children, and almost as frequently in adults. Noncapsulated strains are more frequent than capsulated type b strains which are found in 5% of the samples. Other capsulated strains are rare. Transmission is made easier with close contact (daycare nurseries, home). Colonization is the result of adherence to nasopharyngeal epithelial cells, although characterized adhesion factors cannot be demonstrated for all strains (pili, adhesins, secretory IgA1 protease). Systemic infection is the result of the invasion of pharyngeal epithelium, made easier by upper respiratory tract infection. There is a risk of meningitis for high level bacteremia (> or = 10(5) CFU/ml). Risk factors are: age (child < 5 years), alteration of reticuloendothelial system, agammaglobulinemia. Anti-Haemophilus type b vaccine prevents nearly all infections, and suppresses or sharply reduces colonization.