One has to wonder how a water treatment that is intended to kill bacteria can be completely harmless to humans and animals. Large organisms like humans are actually large collections of cooperating cells, each of which on its own is not radically different from a bacteria. One of the benefits of being a large organism is of course having collective means for cells to survive assaults that kill single-cells living on their own. There is a worry about the chlorination of water that has scientific investigation to substantiate a specific concern - chlorination byproducts. Organic carbons come from plant materials that are impossible to eliminate from public water systems.
Chloroform is found in nearly every public water system in the United States.
Epidemiological studies have found that risk for bladder cancer, colon cancer, and rectal cancer increases with the length of life time one drinks chlorinated waters. Bladder and rectal cancer risk correlates with trihalomethane levels and length of lifetime one drinks chlorinated water. Colon cancer risk correlates with length of lifetime drinking of chlorinated water, but not with trihalomethane levels - some other chlorination byproduct seems to be at work with colon cancer. The practice of adding chlorine to raw water supplies became widespread at the turn of the twentieth century, but it was not until the mid-1970s that Rook and Bellar et. Subsequently, a U.S. Environmental Protection Agency survey reported that chloroform, the most ubiquitous of the THMs, could be found in almost every treated drinking water supply.