At the age of 15 he started to study at the Faculty of Medicine of Rio de Janeiro and in 1892 he graduated as a medical doctor, with a thesis on water as vehicle for the propagation of microbes.

Cruz found that the seaport of Santos was ravaged by an epidemic of bubonic plague that threatened to reach Rio de Janeiro, and engaged himself immediately in the combat of this disease.

The mayor of Rio de Janeiro authorized the construction of a plant for manufacturing the serum against the disease which had been developed at the Pasteur Institute by Alexandre Yersin and coworkers.

In 1902, Cruz accepted the office of director general of the institute and soon expanded the scope of its activities, now no longer restricted to the production of sera but also dedicated to basic and applied research and to the building of human resources.

Cruz was initially successful in the sanitary campaign against the bubonic plague, to which end he used obligatory notification of cases, isolation of sick people, treatment with the sera produced at Manguinhos and extermination of the rats populating the city.

In 1907, on occasion of the 14th International Congress on Hygiene and Demography in Berlin, Cruz was awarded with the gold medal in recognition of the sanitation of Rio de Janeiro.

From the institute he organized important scientific expeditions, which allowed a better knowledge about the health and life conditions in the interior of the country and contributed to the colonization of regions.

His sanitation campaign in the state of Amazonas allowed the completion of construction of the Madeira-Mamoré railroad, which had been interrupted due to the great number of deaths from malaria and yellow fever among the workers.

In 1915, due to health problems, he resigned from the directorship of the Oswaldo Cruz Institute and moved to Petrópolis, a small city in the mountains near Rio.

As a consequence of the short, fruitful life of Oswaldo Cruz, an extremely important scientific and health institution was born, which marked the beginning of experimental medicine in Brazil in many areas.