An outbreak was a sudden increased occurrence of a disease.
In 2366, Jean-Luc Picard noted in his captain's log that "The Enterprise has put in at Rutia IV to deliver medical supplies following an outbreak of violent protests. Although non-aligned, the planet has enjoyed a long trading relationship with the Federation. Now, a generation of peace has ended with terrorist attacks by Ansata separatists who are demanding autonomy and self-determination for their homeland on the western continent." (TNG: "The High Ground")