The central water suppliers do their best to make safe the water that is delivered to your home. They are, however, processing millions of litres per day and delivering this water through miles of pipes of various ages. The water from the tap contains chlorine that has been added to kill bacteria and other contaminants.
The chlorine and filtration systems may not do their job completely. Your water may contain coliform bacteria, giardia lambia, cryptosporidium and hepatitis A. Also water may contain iodine and fluoride both of which be found naturally or artificially. Fluoride is often added by the water companies to help reduce tooth decay.