Both of the individuals who became ill brushed their teeth with untreated water, and one drank tap water at a public building - indicating possible sources of infection unrelated to filtered water. For those using treatment systems greater then one micron, the illness rate was comparable to that of the general population. While the study notes that boiling water is technically the most effective means of treating water infected with Cryptosporidium, it also notes that individuals who drank microfiltered water during the outbreak, including water treated with reverse osmosis, were well protected. It went on to point out that filtering water is a more convenient alternative than boiling. For optimum results, always select a filter with a pore size no greater than 0.4 microns if you expect to protect yourself from bacteria such as cholera, typhoid and e. coli.
Some filters combine a larger pore size with resin which releases iodine on demand.
They are generally effective for bacteria and viruses. All filtration works on the same principle, only the sizes of the pores in the filters are different. The best filtration that is commercially available is filtration by Reverse Osmosis. Pore sizes here are .0006 microns. After this you have various other filtration media which range from 1 micron to 100 commonly known as sediment filters.