The rotten apple injures its neighbour proverbial saying, mid 14th century, often used to mean that one corrupt person in an organization is likely to affect others.
rotten borough a borough that was able to elect an MP though having very few voters. Before the Reform Act of 1832, in which such boroughs were largely disenfranchised, elections in rotten boroughs were rarely contested and the choice of MP was often in the hands of one person or family. The term derives from the borough's being found to have ‘decayed’ to the point of no longer having a constituency.
Rotten Row a road in Hyde Park, extending from Apsley Gate to Kensington Gardens, much used as a fashionable resort for horse or carriage exercise; the name was formerly applied to a number of streets in different towns, although the reason for this is not clear. Now usually called the Row.
something is rotten in the state of Denmark an expression of moral, social, or political corruption, originally as spoken by the ghost in Shakespeare' Hamlet, revealing to his son the story of his brother Claudius' fratricide and usurpation.
See also rot.