Epidemics do not occur by chance. They reveal situations or risk factors—including social determinants—that have created conditions conducive to their occurrence. An outbreak is defined as an excess of cases of a disease in a given location and time period compared to the usual situation. Detection and reactive investigation are the first two essential steps in responding to and controlling outbreaks, and are part of the missions of public health agencies and institutes. An outbreak investigation is based on a methodology structured into ten key steps: confirm the outbreak; define the disease causing the outbreak; identify cases; describe the cases; formulate hypothesis(es) regarding the mode of transmission; test the hypotheses; conduct an environmental investigation; perform microbiological analyses; implement control and prevention measures; and prepare an investigation report. Some of these steps may be carried out in parallel. The investigation requires strong coordination among the various partners involved under the responsibility of an institution (public health institute). Conducted in a responsive manner as soon as the first cases are detected, the investigation enables decision-makers to be presented with appropriate control measures supported by facts scientifically observed in the field. Investigations help improve understanding of how pathogens emerge and spread, knowledge that will help prevent future outbreaks. Social dimensions are important to consider, as the involvement of researchers in the humanities and social sciences has proven useful in improving understanding and the response among affected populations.