SUMMARY. Problems about the pathogenicity of an organism can be classified as: (1) clinical and epidemiological—what diseases does it cause or take part in, and when? (2) research (with prophylactic and therapeutic implications)—how does it do so? Category (1) answers for Haemophilus influenzae began in 1892, with Pfeiffer's claim that it was the cause of influenza; but the true picture of the range of pathogenic activities of this species became clear in the period 1930-1960. Category (2) answers have in general been more recent, and are still far from complete (as is the case with most pathogens); but a fascinating array of contributions has appeared in the past few years.