Hemophilus influenzae type b is a major cause of meningitis in young children. Neurological sequelae, including hearing and vision loss, motor abnormalities, seizure disorders, severe mental retardation, and quadriplegia, may follow the meningitis (Norden, 1982). The case-fatality ratio is approximately 5 percent with adequate treatment, and almost 90 percent without proper care. Other invasive forms of the disease include epiglottiditis, pneumonia, bacteremia, and cellulitis.

Studies indicate that H. influenzae first colonizes the nasopharynx and then penetrates the mucosa. Capsulated H. influenzae strains are responsible for most severe illness, although non-capsulated strains occasionally have been associated with infection. Of the six encapsulated strains, type b is by far the most common cause of invasive disease (Norden, 1982). Host intervention through the production of anticapsular antibodies can prevent disease.

The mechanism by which virulent H. influenzae organisms gain access to the blood is not known, but a bacteremic phase that is generally asymptomatic precedes invasion of the meninges. Whether the organism will go on to cause meningitis depends on its virulence and the immune status of the host. Invasion of the cerebrospinal fluid is followed by the usual symptoms of bacterial meningitis, which if not treated promptly is often fatal.

Nonencapsulated H. influenzae strains, although common, are largely avirulent. There are six serotypes of H. influenzae with immunochemically distinct capsular polysaccharides (Egan et al., 1982). They are identified as types a, b, c, d, e, and f. Almost all invasive H. influenzae disease is caused by type b (Norden, 1982). Thus, a vaccination program can be directed against a single type.