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The genus Haemophilus includes a number of species that cause a wide variety of infections but share a common morphology and a requirement for blood-derived factors during growth that has given the genus its name. Haemophilus influenzae, the major pathogen, can be separated into encapsulated or typable strains, of which there are seven types (a through f including e') based on the antigenic structure of the capsular polysaccharide, and unencapsulated or nontypable strains. Type b H influenzae is by far the most virulent organism in this group, commonly causing bloodstream invasion and meningitis in children younger than 2 years. Nontypable strains are frequent causes of respiratory tract disease in infants, children, and adults.
Other Haemophilus species cause disease less frequently. Haemophilus parainfluenzae sometimes causes pneumonia or bacterial endocarditis. Haemophilus ducreyi causes chancroid. Haemophilus aphrophilus is a member of the normal flora of the mouth and occasionally causes bacterial endocarditis. Haemophilus aegyptius, which causes conjunctivitis and Brazilian purpuric fever, and Haemophilus haemolyticus used to be separated on the basis of their ability to agglutinate or lyse red blood cells, but both are now included among the nontypable H influenzae strains.
