The name is a misnomer because H. influenzae does not cause influenza. Historically, it was once thought to be the cause of influenza, which is why it was given this name.

H. influenzae is a Gram-negative, pleomorphic, non-motile, often encapsulated coccobacillus. A key laboratory feature is that it requires factors from blood to grow in culture:

• Factor X (hemin)
• Factor V (NAD or NADP)

It is associated with both invasive and non-invasive disease. An important virulence factor is IgA protease, which breaks down IgA. Encapsulated strains are more commonly associated with invasive infections.

Meningitis (especially in small children), laryngoepiglottitis or croup, pneumonia, otitis media, chronic bronchitis, sinusitis, and arthritis are common infections caused by H. influenzae.

H. influenzae is a delicate organism, so specimens need careful handling during transport and culture. It may die if samples are refrigerated. Gram stain shows the typical morphology.

Capsular detection can be done by:

• Quellung reaction (the capsule swells when mixed with antisera)
• Latex agglutination
• CIEP (counterimmunoelectrophoresis)
• Fluorescent antibody staining

Culture is done on chocolate agar or on blood agar streaked with S. aureus, which shows satellitism.