H.influenzae was first described in 1892 by Richard Pfeiffer during an influenza pandemic.

The bacterium was mistakenly considered to be the cause of influenza until 1933 when the viral etiology of influenza became apparent.

Taxonomy
Family: Pasteurellaceae
H.influenzae 2 types
- unencapsulated
- encapsulated ► capsular antigens type (a-f)

The most virulent strain is H. influenzae type b (Hib)

Their capsule allows them to resist phagocytosis.

The unencapsulated strains are almost always less invasive; they can, however, produce an inflammatory response in humans, which can lead to many symptoms.

Natural habitats
H. influenzae belongs to the normal flora of the nasopharynx, but rarely in the oral cavity.

They may be present in the vaginal flora.

It is usually the non-encapsulated strains that are harbored as normal flora, but a minority of healthy individuals harbor H. influenzae type b (Hib) encapsulated strains in the upper respiratory tract.

These strains are opportunistic pathogens; that is, they usually live in their host without causing disease, but cause problems only when other factors (such as a viral infection, reduced immune function or chronically inflamed tissues, e.g. from allergies) create an opportunity.

Clinical significance
Naturally acquired disease caused by H. influenzae seems to occur in humans only.

In infants and young children.

H. influenzae type b (Hib) causes bacteremia, pneumonia, epiglottitis and acute bacterial meningitis.

On occasion, it causes cellulites, osteomyelitis, and infectious arthritis.

Vaccination
with Hib conjugate vaccine is effective in preventing Hib infection, but does not prevent infection with unencapsulated strains