Haemophilus influenzae (formerly called Pfeiffer’s bacillus or Bacillus influenzae) is a Gram-negative, coccobacillary, facultatively anaerobic pathogenic bacterium belonging to the Pasteurellaceae family. 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 cause of influenza became apparent, and is still colloquially known as ‘bacterial influenza’. H. influenzae is responsible for a wide range of localized and invasive infections. This species was the first free-living organism to have its entire genome sequenced.