Previously well persons presenting with, or dying from, severe unexplained diseases or syndromes • Higher morbidity or mortality than expected for a common disease or syndrome • Cases of known aetiology occurring in an unusual setting, population or season, or with an atypical clinical presentation, or with unusual or atypical strain • Indistinguishable molecular and genetic characteristics of agents detected in temporally or spatially distinct sources • Presence of a large epidemic or multiple simultaneous outbreaks in discrete populations • Deaths or illness among animals with the potential for zoonotic disease transmission to humans • Illness affecting a key sector of the community (e.g.