General Information: Small, nonmotile, Gram-negative bacterium whose only natural host is human. It lacks an important fimbrial gene cluster that is important for virulence as compared to type b strains and was the first microbe to ever be sequenced. A group of organisms that are either obligate parasites or commensal organisms found in animal mucous membranes. Almost all species require the presence of important growth factors found in the blood of their hosts, including either X factor (protoporphyrin IX or heme) or V factor (nicotinamide adenine dinucleotide (NAD or NADP)). This organism was first isolated in the 1890s during an influenza pandemic by Pfeiffer, and was originally thought to be the source of influenza, although later it was shown to be a secondary pathogen and may be synergistic with the influenza virus. This bacterium is one of the leading causes of meningitis in young children, and it may also cause septicemia, otitis media (inflammation of the middle ear), sinusitis (inflammation of the sinus cavity) and chronic bronchitis. It is highly adapted to its human host and typically lives in the nasopharynx and is a major cause of lower respiratory infections in infants and small children in developing countries (type 1b strain), although vaccine use has resulted in the decline of infections. The encapsulated organism can penetrate the blood and avoid both phagocytosis and complement-mediated lysis. All known strains produce neuraminidase and an IgA protease as well as fimbrial adhesins for attachment.