One Rotten Apple Spoils the Whole Barrel
This means that the influence of one bad person can mess up or taint the entire group. The etymology is rooted in the science that an apple produces a gas, ethylene, that ripens the fruit. When a fruit is overripe or rotten, it produces more of this gas and spoils any of the other fruit in the barrel.
When you store fruits together, the ethylene each piece emits encourages the others around them to ripen further. It’s contagious.
Quick tip: to ripen an avocado in a short amount of time, stick it in a paper bag with an apple overnight.
It really only takes a single apple to start a domino effect that will ruin the others in the bunch. The same is said to be true with people. Chaucer was quoted saying that a "rotten apple ruins its neighbors.” The proverb originally was "One mouse dropping ruins the whole pot of rice” but evolved over time.