Haemophilus influenzae, a small gram-negative nonmotile parasitic bacterium that occurs in two forms, encapsulated and nonencapsulated, and in six types, a, b, c, d, e, and f. Almost all infections are caused by encapsulated type b organisms. H. influenzae is found in the nasopharynx of approximately 75% of healthy children and adults. In children and in debilitated older people, severe destructive inflammation of the larynx, trachea, and bronchi may result from infection. Subacute bacterial endocarditis, purulent meningitis, and pneumonia also may be caused by it. Secondary infection by H. influenzae occurs in influenza and in many other respiratory diseases. Several H. influenzae B conjugate vaccines are available.
