Genus of tiny rod-shaped bacteria .
All are strict parasites occurring in the respiratory tracts of warm-blooded animals, including humans, and in certain cold-blooded animals. Some require oxygen, others do not. H. influenzae causes severe bacterial diseases in young children, including meningitis, otitis, and pneumonia. Another species of Haemophilus causes a sexually transmitted disease in humans known as chancroid, or soft chancre . Yet another causes secondary infection in persons with influenza.