Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, is a Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe.
H. influenzae was mistakenly considered to be the cause of influenza until 1933, when the viral etiology of the flu became apparent; the bacterium is colloquially known as bacterial influenza. Still, H.
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