One scabby sheep is enough to spoil the whole flock.
More French Proverbs:
To every saint his candle.French
One catches more flies with a spoonful of honey than with twenty casks of vinegar.
French
Don't teach fishes to swim.
French
He who is master of his thirst is master of his health.
French
One meets his destiny often in the road he takes to avoid it.
French
He who wants to conquer lechery must flee from it.
French