Light micrograph (LM) of Haemophilus influenzae (formerly called Pfeiffer's bacillus or Bacillus influenzae), a nonmotile, Gram-negative, rod-shaped bacterium. H. influenzae was mistakenly considered to be the cause of influenza until 1933, when the viral etiology of the flu became apparent. Still, H. influenzae is responsible for a wide range of clinical diseases. Gram stain, magnification: 1000x at 35mm.