Haemophilus influenzae?is a bacteria that has encapsulated (typeable) or unencapsulated (nontypeable) strains. Encapsulated strains express one of six antigenically capsular polysaccharides (types a, b, c, d, e, or f). Historically, type b (Hib) was the most common type to cause invasive disease, particularly in young children.?H. influenzae?colonizes the upper respiratory tract of humans and is transmitted person-to-person by inhalation of respiratory droplets or by direct contact with respiratory tract secretions.

Encapsulated?H. influenzae?nontype b strains, particularly type a, can cause invasive disease similar to Hib disease. Nontypeable strains also can cause invasive disease but more commonly cause mucosal infections such as otitis media, conjunctivitis, and sinusitis. Vaccines are only available for?H. influenzae?type b; Hib vaccines do not protect against disease caused by any other?H. influenzae?strains.