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There is an incubation perioid usualy about a week up to two weeks where the duck is infected but not showing symptoms yet. If the ducklings died as soon as they came in contact with the virus it would not have much of a chance to spread.

Sounds like it could have been duck viral hepatitis or duck viral enteritus. Both are marked by sudden death and mortality rates of 90%. Hopefully your new ones don't get it but you wont know for another week or two.