Haemophilus influenzae (formerly called Pfeiffer's bacillus or Bacillus influenzae) is a Gram-negative, coccobacillary, facultatively anaerobic capnophilic pathogenic bacterium of the family Pasteurellaceae. H. influenzae was first described in 1892 by Richard Pfeiffer during an influenza pandemic.The bacterium was argued by some to be the cause of influenza until 1933, when the viral nature of influenza was firmly established, and infections are still colloquially known as bacterial influenza. [Source: Wikipedia ]