The chlorination of public drinking supplies was originally met with resistance, as people were concerned about the health effects of the practice. The use of chlorine has greatly reduced the prevalence of waterborne disease as it is effective against almost all bacteria and viruses, but even to this day there are those who wish to move to other purification systems. They point to the production of dangerous by-products in the water when chlorine reacts with naturally occurring organic compounds found in the water supply. The most common of these are trihalomethanes (THMs), which include chloroform. Another method to purify water is to use ozone and some municipalities have switched to that method, but ozone breaks down quickly so smaller amounts of chlorine are still added to these systems to ensure that water downstream from the treatment facility continues to be safe.