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Main offices of the General Directorate of Public Health, at 128 Rua Resende (now the National Cancer Institute). Cover of the publication Archivos de Hygiene, year I, no. 1, 1927

When Oswaldo Cruz was appointed to head the General Directorate of Public Health in 1903, yellow fever was the main sanitary problem in the nation’s capital. In 1902 alone, nearly one thousand people in Rio died of the disease. It was mostly because of the illness that the city had earned the unsavory reputation of being a “grave for foreigners”.

Until the late nineteenth century, there was a relative consensus about the origin of yellow fever. The most widely accepted thesis was the miasma theory, according to which infectious disease was caused by foul air that emanated from swamps and rotting organic matter. It was also believed that contact with the ill was the form of contagion.

These explanations began to be questioned at the turn of the century, when a commission of US military doctors in Cuba successfully tested Cuban physician Juan Carlos Finlay’s hypothesis that yellow fever is transmitted by the mosquito Stegomyia fasciata (today known as Aedes aegypti, which also transmits dengue fever).

As a proponent of Finlay’s theory, Cruz based his entire campaign on battling the mosquito. Structured along military lines, the campaign got underway in April 1903 with the establishment of the Specific Yellow Fever Prophylaxis Service. The city was divided into ten sanitary districts, each under the jurisdiction of the health delegacies that were responsible for collecting reports on the sick, issuing fines, and ordering owners to reform or demolish unhealthy buildings.

Cruz created sanitary brigades to eliminate the breeding grounds of mosquitoes; these teams were popularly called “mata-mosquitos,” or “mosquito killers.” Armed with insecticides and the appropriate tools, the brigades made daily rounds of streets and houses, disinfecting every spot where insect larvae might be found. The isolation and expurgation division was responsible for cleaning up houses in regions of foci and also for isolating the sick in their homes or at hospitals.

To publicize his initiatives, Cruz released a series of educational pamphlets to the press. Entitled “Advice for the Public” (Conselhos ao Povo), the material explained the measures being adopted. At a time when most people were illiterate and many doctors did not believe Finlay’s theory and were recalcitrant about mandatory reporting to public health about their patients, the effort was not enough to overcome resistance to the campaign.

Challenges notwithstanding, the epidemic gradually waned. This fact was Cruz’s chief argument for silencing his adversaries. Another vital ally was the arrival in Rio de Janeiro of a mission from the Pasteur Institute in Paris to study the validity of the “Havana theory”—as Finlay’s hypothesis was dubbed in Brazil. Composed of physicians Émile Roux, Paul-Louis Simond, and Alexandre Salimbeni, the mission arrived in Rio in November 1901 and stayed for four years. Its final report was favorable to the new prevention strategy.

By early 1907, there was no more talk of yellow fever epidemics in Rio de Janeiro. Cruz had kept his promise. In March of that year, he could at last write to President Afonso Pena: “Thanks to the firm stance and will of the government, yellow fever is no longer assailing the capital of the Republic in epidemic form”.