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First-class rail pass issued by the São Paulo Railway Company, used by Cruz to travel between São Paulo and Santos, where he played a key role in efforts to control the bubonic plague outbreak. São Paulo, Oct. 23, 1899

In early 1904, when the yellow fever campaign was starting to record its first successes, Oswaldo Cruz launched a new battle: the fight against the bubonic plague.

Of the three major campaigns that Cruz led in Rio de Janeiro while head of the General Directorate of Public Health, the one against the plague met with the least resistance (the third would be against smallpox). After all, hardly anyone disagreed with the notion that the malady was transmitted by the bite of fleas that had been infected by rats, in turn contaminated with the bacteria Yersinia pestis—the bacillus discovered by Alexandre Yersin and Shibasaburo Sato in 1894. Serum therapy and vaccination were already well established in the case of the plague.

In addition to promoting the vaccination of those living in more heavily infected areas, Cruz relied on mandatory reporting to guarantee the isolation of the sick and treatment with serum made at the Federal Serum Therapy Institute. Concomitantly, he promoted a broad-based campaign to rid the city of rats.

Staff assigned to this mission were obliged to kill at least 150 rats a month or risk job loss. Anyone who exceeded the minimum quota was rewarded with an extra 300 reis per animal. The General Directorate of Public Health also began buying rats at the price of 200 reis per corpse.

Since anyone could sell rats to the government, a new occupation soon appeared in the city: “ratoeiros”—literally, ratters—men who went around town buying up rats at a low price and selling them to the General Directorate of Public Health. There were even some who raised rats at home for this very purpose and also those who traveled to other cities to find them. In a short while, the “ratter” trade was doing a booming business.

The “war of the rats” became the brunt of jokes and mockery among the residents of Rio, inspiring countless cartoons, caricatures, short stories, and even songs. Of course, the favorite target and key figure in these attacks was Cruz.

Although deaths from the bubonic plague declined at a slower pace than had been the case with yellow fever, the mortality rate for the plague also dropped off substantially in Rio, attesting to the success of Cruz’s campaign. In 1903 the death rate had reached 48.74 per 100,000 inhabitants, and it fell dramatically over the next five years. By 1909, the year Cruz left the General Directorate, the rate hit its nadir: 1.73 per 100,000. Once again, Cruz had won the battle.