Haemophilus influenzae is a small, nonmotile Gram negative bacterium found naturally in the nasopharynx of approximately 75 percent of healthy children and adults. The presence of the type B polysaccharide capsule is the major factor in virulence. Encapsulated organisms can penetrate the epithelium of the nasopharynx and invade the blood capillaries directly. Their capsule allows them to resist phagocytosis and complement mediated lysis in the the nonimmune host and cause bacteremia and acute bacterial meningitis in infants and young children. Occasionally epiglottitis cellulitis, osteomyelitis, and joint infections can be caused.
