Oswaldo Gonçalves Cruz (1872-1917) was a Brazilian microbiologist, epidemiologist, and public health officer who founded experimental medicine in Brazil and directed controversial programs to eradicate yellow fever and smallpox from Rio de Janeiro.

Oswaldo Cruz was born in the province of São Paulo, the son of a doctor. He completed medical school at the age of 20, perhaps as much because of the elementary nature of medical instruction then provided in Brazil as because of his brilliance. In 1896 he went to Paris, where he worked at the Pasteur Institute for 3 years. Cruz returned to Brazil as the bearer of an entirely new outlook on medical problems. His understanding of modern principles regarding contagion was perhaps not unique even in Brazil, but he was exceptional in his ability to surmount the political obstacles to the application of this understanding to public health. He almost immediately demonstrated these abilities in the coastal city of Santos,.

In 1902 Cruz became the Brazilian director general of public health. Brazil's progress and effort to secure international respect had so far been severely hampered by the frequent epidemics that ravaged the population, discouraged immigration, upset the normal patterns of trade, and debilitated both workers and managers. With the President's backing, Cruz launched a vigorous campaign aimed at imposing sanitary standards first of all upon the capital city. He especially worked to eradicate the mosquito responsible for the transmission of yellow fever. Simultaneously he pushed through the Brazilian congress a law requiring compulsory smallpox vaccination of all citizens.

These programs encountered the resistance of a superstitious and conservative population. Alarmed by these newfangled ideas and the invasion of their privacy and individual freedom, the people were easily manipulated by opponents of the regime: urban riots and even an unsuccessful military revolt were the result. The President, however, continued to give Cruz his full support, and the campaign was successful. As of that time Rio de Janeiro ceased to be a synonym for epidemic disease.

Meanwhile, Cruz also became director of the newly formed Institute of Experimental Pathology. His energetic and progressive leadership soon made it world-famous in the field of tropical medicine. He personally conducted field experiments in the upper Amazon and began the long process by which malaria was effectively restricted in Brazil. His career was cut short by Bright's disease.