[The "blow", the 1918 pandemic in Mexico]

After eight years of a civil war which devastated the country, Spanish flu, one of the worst pandemics in the history of humankind, arrived in Mexico in October of 1918. This article discusses its arrival to the port of Veracruz in ships coming from Habana and New York City; its dissemination from the Gulf of Mexico area to the rest of the country, including Mexico City; and the responses of both federal and local health authorities. Two events associated to this pandemic are particularly relevant, in addition to the high number of deaths: the testing of the sanitary dispositions added to the 1917 Mexican Constitution and the extraordinary role played by civil society organizations.