The water cycle transports a plethera of microorganisms, and many of them are harmful for human beings. These include bacteria, protozoa, viruses, and lavae of high-ordered parasytes. Some of the aforementioned are Giardia, Cyptospordium, Shigella, E. coli, hepatitis A virus, and enterovirus.
Waters can also carry chemical wastes such as pesticides, and heavy metals dissolved in them. It is common to hear about acid rain as a result of pollution, and it originates as a result of mixing sulphur dioxide and nitrogen oxide with water molecules in the atmosphere.
There are several technological improvements over the past 100 years that have made the availability of clean water to communities a lot easier, and not only depend on boiling water. Filtration is the most common practice, it traps most unwanted particles and microorganisms, but it should be accompanied with chemical disinfection, which uses halogens to oxidize essential celluar structures and enzymes. In the present, chemical disinfection can even be achieved with adding a pill to a specific volume of water.
