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A page from Oswaldo Cruz’s lab notebook where he recorded the results of an analysis of a sample of organic material from a person living in the Vale do Paraíba to prove the existence of a cholera morbus epidemic. Dec 5, 1898

In 1894, Oswaldo Cruz had his first major hands-on experience in public health. The fifth cholera pandemic of the nineteenth century—which began in 1891 and lasted through 1896—hit São Paulo that year and quickly spread into Rio de Janeiro along the Paraíba Valley, the geographic heart of São Paulo’s coffee economy. In its battle against the disease, the government established the Federal Sanitary Institute, born of the merger of the Bacteriology Laboratory (successor of the National Institute of Hygiene) and the Sanitary Directorate.

Francisco Fajardo, head of the new institute’s laboratory, invited Cruz and Eduardo Chapot-Prévost, instructor of histology at the School of Medicine, to join the commission assigned to ascertain the precise nature of the epidemic. Local doctors claimed the illness was a form of dysentery caused by a variety of non-contagious factors and denied it might be cholera morbus, as diagnosed in Santos at the newly inaugurated Bacteriological Institute of São Paulo, headed by Adolfo Lutz.

The commission traveled to the region and collected material, which was submitted to bacteriological testing at the laboratories in Cruz’s and Chapot-Prévost’s own homes; the tests detected the presence of Vibrio comma, the comma-shaped bacterium identified by Robert Koch in 1893. This confirmation of the diagnostic results obtained by the São Paulo laboratory prompted the government to implement a wide-ranging sanitation program in strategic areas of Rio de Janeiro, São Paulo, and Minas Gerais. It can be said that the campaign in the Paraíba Valley launched Cruz’s career as a sanitarian. More than any other, it would be this facet of his professional life that would make his a household name in Brazil, even today.