Paul uses this proverb to mean that a small part of something affects the whole thing. In this case, one person who is sinning can harm the entire Christian community. (See: [[rc://en/ta/man/translate/figs-proverbs]])
Here Paul uses a rhetorical question to teach his audience. Alternate translation: "You know that a little yeast ... loaf." or "A little yeast ... loaf." (See: [[rc://en/ta/man/translate/figs-rquestion]])