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Water treatment by reverse osmosis is sometimes referred to as hyper-filtration as it relies on a semi-permeable membrane to filter out contaminants. Membranes used for water treatment act like a microscopic sieve to strain contaminants out of water.  Reverse Osmosis membranes allow water molecules to pass through, but not larger compounds. As water is a very small molecule, almost everything else can be removed leaving pure water.

In the attached animation the relative sizes and removal of various organic and inorganic contaminants that may be found in water is demonstrated, starting from the largest microbiological contaminants known as Protozoa.

Protozoa include organisms such as Giardia and Cryptosporidium that cause gastroenteritis amongst other, usually non-fatal symptoms. If a water molecule was the size of a squash ball or a table-tennis ball, a protozoan would be the size of a small mountain.

Bacteria include many disease-causing organisms, including many water-borne diseases found in third world countries (for example cholera). If a water molecule was the size of a squash ball, a bacterium would be the size of a building.

Viruses range in human impact from causing the common cold to spreading HIV. They are particles that usually only infect people and are passed from person to person. If a water molecule was the size of a squash ball, a virus particle would be the size of a car.

Organic and inorganic molecules do not cause disease in the same way as microorganisms and are found naturally in water. However if ingested consistently in large quantities or over a long period some can cause unpleasant taste, toxicity or in the extreme case cancer. Organic molecules include chemicals present in plastics and pharmaceuticals and are generally larger than inorganic molecules. Salt is the most common form of inorganic molecule.  If a water molecule was the size of a squash ball, an organic molecule would be the size of a soccer ball and an inorganic molecule would be the size of a cricket ball.

Therefore, as all contaminants are larger than water molecules, essentially all contamination of water is removed by passing the water through a reverse osmosis membrane.