• Haemophilus influenzae is a small Gram-negative bacillus.
• It is nonmotile.
• Facultative anaerobe (def).
• Fastideous growth needs. Requires enrichments for growth.

Habitat

• Mucous membranes of the respiratory tract in humans.

Source

• The patient's own mucous membranes or transmitted patient-to-patient.

Epidemiology

• Haemophilus parainfluenzae and nonencapsulated H. influenzae typically colonize the upper respiratory tract in humans within the first few months of life. These bacteria typically cause sinusitis, otitis media (def), bronchitis(def), and pneumonia (def).
• Encapsulated H. influenzae, primarily H. influenzae type b, is uncommon as normal flora of the upper respiratory tract but can be a common cause of serious infection in children.
• Until immunization of children against H. influenzae type b became routine in developed countries, this bacterium was the most common cause of pneumonia, septicemia(def), meningitis (def), and epiglottitis (def) in children under the age of four. Immunization has reduced the incidence of systemic infection by this bacterium 95%.

Clinical Disease

• Haemophilus influenzae does not cause influenza. Influenza is a viral infection.
• Haemophilus parainfluenzae and nonencapsulated H. influenzae typically cause sinusitis, otitis media (def), bronchitis (def), and pneumonia (def).
• H. influenzae type b is the most common cause of pneumonia, septicemia (def), meningitis (def), epiglottitis (def), and cellulitis in children under the age of four who are not immunized.

From Haemophilus influenzae Infections, by Mark R Schleiss, MD, Associate Professor, Department of Pediatrics, Division of Infectious Diseases, University of Cincinnati and Children's Hospital Research Foundation.