An outbreak is the sudden appearance and rapid spread of something harmful, especially a disease. When doctors notice many people in a town getting sick with the same illness at the same time, they call it an outbreak. The flu might cause an outbreak at your school, spreading from classroom to classroom until many students are home sick.

Outbreaks happen when diseases spread faster than usual in a specific area. A few people getting colds is normal, but when dozens of cases appear within days, that's an outbreak. Health officials work quickly to stop outbreaks before they grow into epidemics (affecting a whole region) or pandemics (spreading across countries or continents).

The word can also describe other sudden, unwanted events. An outbreak of fire means flames have started spreading rapidly. An outbreak of fighting means conflict has suddenly erupted. There might be an outbreak of laughter in a classroom (though that's considerably more welcome than an outbreak of illness). The word always suggests something bursting forth quickly and spreading: sudden, intense, and often requiring an urgent response to contain it before it gets worse.