One of the miracles of modern society is the abundant supply of clean drinking water available in every home and business. All you have to do is turn on the tap to drink clean, germ-free water. We take this miracle completely for granted, but if you ever travel to a country that doesn't have a good water system, you'll quickly learn to appreciate the incredible convenience of our water system.
What would happen in the United States if the water supply were to become contaminated by some sort of bacteria? Bacteria have a difficult time getting into drinking water because the water system is designed to keep them out. A typical water system pumps raw water from a river or a lake, removes the sediment in a settling tank, filters the water with a sand filter and then decontaminates it with chlorine, ozone and/or ultraviolet light to kill any bacteria that remain. The result is clear, healthy drinking water completely free of germs.
There are times, however, when the purification systems break down. This is especially common is small water systems in rural areas, where the water isn't tested or monitored regularly. But it can also happen in big cities. The worst case so far happened in Milwaukee in 1993. A protozoa called cryptosporidium got into the water system, killing dozens of people and sickening approximately 400,000. The reason this protozoa was able to infect the water supply is because it's small and therefore resistant to filtering, and because chlorine isn't very effective against it. After the incident, Milwaukee installed an ozone system in addition to the chlorine system to guard against future occurrences.
In smaller water systems, especially those operating off of wells in rural areas, E. coli contamination of the well and poor monitoring can lead to problems. E.Coli bacteria are killed by chlorine, but the concentration has to be high enough and the exposure time long enough for the chlorine to be effective. Certain strains of E. coli are deadly, especially to children and senior citizens.
So the answer to the question, "What if the city water supply gets contaminated?" is "It could infect half the people in the city." The solution to this problem is constant and careful monitoring of the purification process, along with the use of several different purification systems to handle different kinds of contamination.