Rare said
Pepperm1nts, I already knew about the Vaccine Revolt of 1904 and the Revolt of the Lash in 1910. The Vaccine Revolt was caused by the approve the Mandatory Vaccination Law. While the Revolt of the Lash was caused by the fact, that black people were forced to enter the navy, where they faced widespread discrimination and other shit. Both could be stop in a year or so.
The Vaccine Revolt thing isn't that simple though. Yeah, it can be fixed in one successful turn (probably), but it wasn't as simple as "the people were angry about vaccines!"
From the wiki:
At the beginning of the 20th century the city of Rio de Janeiro, then capital of Brazil, although noted for beautiful palaces and mansions, also suffered from serious inadequacies in infrastructure, including insufficient water and sewer systems, irregular garbage collection, and densely populated tenements.
In this environment many illnesses proliferated, including tuberculosis, measles, typhus and leprosy. From time to time epidemics of yellow fever, smallpox and bubonic plague occurred. Between 1897 and 1906, 4,000 European immigrants died in Rio de Janeiro from yellow fever alone.
In short, you have some infrastructure to fix.