Abstract This article examines a smallpox epidemic which killed 1% of the population of Porto Alegre in 1874. Through extensive documentary research and comparison with data from those who died, we problematize why smallpox manifested as an epidemic in the city. Maps showing vaccination in the years preceding the outbreak reveal that only low levels of the population of Porto Alegre participated in prevention efforts, and the benefits of these efforts were ignored by the different social groups which were interconnected within the city. As sick soldiers arrived from other places, the disease spread rapidly through the city and caused the death of hundreds of people.

In the pre-vaccination era in Sweden, smallpox was estimated to have been responsible for 2,000 deaths per million people every year; in other terms, this meant that approximately 0.2 percent of the entire population (or one in every 500 people) would die of smallpox annually. From looking at other data sets, we know that this figure was as high as 7,200 deaths per million in some years, as individual epidemics regularly devastated large portions of the population. When Jenner's findings on vaccination were adopted by the scientific community in Europe, Sweden was one of the first countries to begin vaccinating infants on a large scale. This is reflected in the considerable decline of smallpox deaths in the early nineteenth century, where the number of deaths fell to just 623 annual smallpox deaths per million in the first decade. By the middle of the century, when vaccination was made compulsory by the Swedish government, it dropped even further, to less than ten percent of the pre-vaccination death rate. In the last two decades in the nineteenth century, when Swedish authorities began penalizing parents for not vaccinating their children, the smallpox death rate fell to just two deaths per million people, and Sweden reported its final endemic (naturally occurring) case of smallpox in 1895; making it the second country in the world (after Iceland in 1872) to successfully eradicate the disease.

Error is in deaths, a is the total number of infected people, r is the maximum rate of infection (infected per day), and c is the day this maximum occurs.