Once upon a time there was a farmer who hired a dog to watch over her sheep. One day the farmer heard a terrible commotion coming from the field and rushed outside to see a wolf devouring a sheep. “What are you doing?!” raged the farmer to the dog....

“I’m protecting the sheep” said the dog.
“No you aren’t!” Cried the farmer, “A wolf is eating the sheep right now!”
“True” said the dog, “but that is because the wolf is hungry. Once the wolf has eaten its fill, it will go away and the sheep will be safe.”
At that moment...

..the wolf killed another sheep.
“I hired you to chase away the wolves,” said the angry farmer.
“I could do that,” said the dog, “but the wolf would still be hungry and I would just have to keep chasing it away. Better to let it eat the old and the slow.”
Right then...

...the farmer decided to replace that dog with one that would protect the sheep from the wolf instead of pursuing the strategy of trying to produce herd immunity from wolves.