means A sudden, often rapid occurrence or spread of something unwanted — usually a disease, but also war, violence, or any trouble that erupts all at once.

from A straightforward English compound of "out" plus "break" — the sense of something breaking out, bursting forth from confinement. The verb phrase "break out" is old, and the noun "outbreak" follows the same logic as kin like "outburst" and "outflow." The word carries the image of pressure suddenly released — something that was contained, now loose. Its strong association with disease is a later specialization; originally it could describe any eruption, from a fire to a rebellion.