A genus of Gram-negative aerobic nonmotile parasitic rod-like bacteria frequently found in the respiratory tract. They can grow only in the presence of certain factors in the blood and/or certain coenzymes: they are cultured on fresh blood agar. Most species are pathogenic: H. aegyptius causes conjunctivitis, and H. ducreyi soft sore (chancroid). H. influenzae is associated with acute and chronic respiratory infections and is a common secondary cause of influenza infections.