A common nose and throat resident that can also cause ear, lung, bloodstream and meningeal infection. Explore its illustrative 3D model, human context, and sources.
H. influenzae lives in the nose and throat of many healthy people and usually causes no harm. Some strains carry a polysaccharide capsule; capsular type b (Hib) caused most invasive childhood disease before Hib conjugate vaccines, while unencapsulated nontypeable strains are the ones most often carried. Despite the name it does not cause influenza: that association was proposed during an 1892 outbreak investigation and corrected once influenza was shown to be viral in 1933. Reported infections range from ear infection and bronchitis to pneumonia, bloodstream infection and meningitis, and CDC states that symptoms depend on which part of the body is infected.
Most carriage causes no illness. Hib vaccines prevent type b disease but not disease caused by other H. influenzae, so capsular type matters. A positive nose or throat sample is not a diagnosis of disease, and risk differs by age, medical condition and vaccination status.
An original illustrative 3D model of one short, rounded, encapsulated type b (Hib) coccobacillus. H. influenzae is pleomorphic: the documented 0.3-0.5 by 0.5-3.0 micrometre range includes longer forms, so this single outline is not a species-wide fixed shape or a measured strain reconstruction. The translucent type b polysaccharide capsule is not assigned to nontypeable strains. The Gram-negative outer membrane, thin peptidoglycan and plasma membrane are separated for inspection, not to scale. No flagella, endospores or default pili are depicted; pilus expression varies by strain and state. Surface finish, colours, molecule sizes, cell contents and division stages are schematic. Compared with CDC PHIL photomicrograph 23029, PHAC morphology, CDC Pink Book and Doern & Buckmire's capsulated type b microscopy study on 2026-09-18. Independent scientific review is pending.
Source check: 2026-09-19. Educational content; independent scientific review remains pending.