The following is from the book, Mental Floss Presents: Condensed Knowledge. An excellent book! I'd like to cite the following paragraph to illustrate its readable style. A good compendium of general-purpose knowledge will be reasonably thorough without sacrificing readability.
The power of good plumbing is incredible. While good sanitation can't rid the world of all diseases, it has made a huge dent in epidemics caused by water-borne infectious diseases. Around 1700 BCE indoor plumbing came to royalty on the island of Crete. A millennium later Hippocrates (ca. 460-ca. 377 BCE) saw the importance of clean water and recommended that ancient Romans boil water to remove pollutants. The Romans hadn't a clue about bacteria and viruses, but we do. An easy way to pass bacterial diseases like cholera and intestinal infections is to come in contact with things that "pass through" people and animals. The moral: Don't wipe with the hand that feeds you.
Also see: Immunogenic