We’ve all heard the saying "one bad apple spoils the whole bunch," and have probably seen instances where it does apply to people, but does it actually happen with fruit?
Yes. As they ripen, some fruits, like apples and pears, produce a gaseous hormone called ethylene, which is, among other things, a ripening agent. When you store fruits together, the ethylene each piece emits prods the others around it to ripen further, and vice versa. (Fun tip: Want to quickly ripen an avocado? Stick it in a paper bag with an apple overnight.)
The riper a piece of fruit is, the more ethylene it produces, and overripe fruit gives off even more ethylene, eventually leading to a concentration of the gas that’s enough to overripen all the fruit. Given the right conditions and enough time, one apple can push all the fruit around it to ripen—and eventually rot.
Additionally, an apple that is infested with mold will contaminate other fruit it's stored with as the mold seeks additional food sources and spreads. In both cases, it actually does take just one single apple to start a domino chain that ruins the rest of the bunch.