Although the passage was not provided, Jonathan Swift used irony and sarcasm to make harsh criticism of rich people who exploit the poor.
This passage was probably taken from "a modest proposal" which was one of Swift's most notable works. In this brilliant satire, Swift addresses the misery that plagued eighteenth-century Ireland and mocks the eternal neglect of the rich in relation to the poor, always seen more as animals than as people. Very grateful criticism of the inhuman social policies of the time.
Answer: The author might be suggesting that the bacteria and diseases from the cesspool are leaking into their water source and into the shared water pump water, spreading the disease.