This paper gives a brief overview of the impact of the 1918-1919 flu and the polio epidemics and the responses to them, to show how they were a challenge and also an opportunity to draw attention to the role that could be played by health professionals specialising in bacteriology, and later virology, in international centres; and where new prophylactic and therapeutic resources -serums and vaccines -could fit into this new approach. Using medical, scientific and legislative sources, a selection from the general press, and archival and printed documentation of the WHO about the social history of medicine and, particularly, the history of the disease, it shows the important role of the training obtained by health professionals in international research centres in the management of both epidemic events, in the modernisation of Spanish science and health before the civil war, and in the post-war reconstruction.