One good thing about protozoa is its large size. This makes it easy to filter out. Many of the leading filters on the market filter particles as small as 0.2 microns, meaning that they will catch protozoa. Interestingly, protozoa are immune to chemical purification treatments because of their hard, protective shell. This is one reason that many purifier manufacturers choose to also have some sort of filter element on their purifiers.
Bacteria are the next largest particulates in water that can cause illness.
Salmonella and E. Coli are some examples of bacteria. They are approximately 0.2 microns and larger in size, meaning that filters will be able to catch them as long as the filter pores are at least as small as 0.2 microns. Purifiers can also kill bacteria. Without catching bacteria a person will be in danger of getting Cholera and Typhoid Fever. The next smallest particulates in the water are viruses.