The other reason is that its contents may contain the germs of serious diseases, particularly typhoid fever and other bowel troubles.

These germs and their poisons would usually be destroyed by the bacteria of the soil, if not poured out in too large quantities; but in the privy vault they escape their attack, and so are carried on with the slow leakage of water into the well; then those who use that water are very liable to have typhoid fever and other serious diseases.
Early Methods of Prevention.

On account of these filth-dangers, it began, a century or so ago, to be the custom in cleanly and thoughtful households to provide, first, ditches, and then, lines of pipes, made out of hollow wood or baked clay, and later of iron, called drains, through which all the watery parts of household wastes could be carried away and poured out at some distance from the house.

Then toilets, or flush-closets, were built, and this kind of waste was carried completely away from the house, and beyond danger of contaminating the wells.
How Streams were Contaminated.

For a time this seemed to end the danger, as the waste was soaked up by the soil, and eaten by its hungry bacteria and drunk up again by the roots of plants.