The defintion of outbreak  is clear for me : it is the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season. outbreaks of single cases is also clear for me .  What is vague for me is  as follows... eg : if  the regular vaiation for  a disease is about 100 -200 per year when to say that it is an outbreak ? if it exceeds 250? or 300 or 400?  how to confirm that this is an outbreak ? at what level ? and on what basis? is there any reference or general rule?

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