A genus of aerobic to facultatively anaerobic, nonmotile bacteria (family Brucellaceae) containing minute, gram-negative, rod-shaped cells that sometimes form threads and are pleomorphic. These organisms are strictly parasitic, growing best, or only, on media containing blood. They may or may not be pathogenic. They occur in various lesions and secretions, as well as in normal respiratory tracts, of vertebrates. The type species is Haemophilus influenzae.