Gregorio Marañón y Posadillo (1887-1960), played a leading role in the birth of endocrinology in Spain as is well known. However, his medical work included other important and significant fields. Thus, it was especially in the 1910s and 1920s, when Marañón dealt with the social-sanitary situation in madrid probably due to his professional attachment to the treatment of several infectious diseases and epidemic outbreaks. Actually, since 1911 onwards, he was in charge of the wards of infectious diseases in the Hospital General de Madrid where he had the opportunity of treating an important number of patients suffering from this type of pathology and, as a consequence, in the following years he published several articles in medical journals and presented in the Royal Academy of Medicine in Spain, some reports on infectious diseases and the Spanish health and social conditions at the time. This paper try to analyze this field of Marañón's social and scientific activity.