To remove bacteria (see the attached figure) your filter must be able to remove particles 0.2 microns or larger (a 0.2 micron absolute filter does this; absolute implies the largest size particle the filter lets through, opposed to a 0.2 micron nominal filter, which implies the smallest particle the filter is capable of removing; there is a big difference between these two definitions!). Similarly, to remove viruses the filter must remove particles 0.01 microns or larger. Ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) are capable of doing this; however, RO also reduces the good minerals.