In the context of renewed interest in the 1918-1919 flu pandemic as a result of the development of COVID-19, this paper presents a reflection from the point of view of the social history of medicine on the transformation that the erroneously named “Spanish flu” brought to the working environment of health professionals in Spain, combining what happened in Madrid and in the country as a whole. Using as our sources monographs and the principal professional scientific journals of physicians, pharmacists and veterinarians, together with a selection from the general press and documentation from both houses of Parliament, we show how the daily activity of doctors was marked by the escalation of their clinical work as well as a considerable deployment of laboratory research in order to establish the aetiology of the flu and to prepare specific therapeutic and prophylactic measures. We show the difficulties this work encountered, and the interest of pharmacists and veterinarians in reproducing this new aspect of science and competing with doctors to achieve greater involvement in the bacteriological fight against infectious diseases. The paper also points out how, faced by the difficulty of controlling the pandemic, a section of the medical profession turned to the socioeconomic inequalities (poor working and living conditions, overcrowding, shortage and cost of food, medicine and basic necessities) in the working population and shortcomings in health care to explain its extreme seriousness, and proposed scientific medical and general measures to correct the situation in the future.