Well, since there is science involved here, I feel I should mention a couple of things.

Filter purification systems work by forcing water through what is essentially a very fine sieve. The size of the holes (pores) varies depending on the filter. In Sawyer filters, they are 0.1 microns (a thousandth of a millimetre) in diameter. This means that they will remove things like cysts and bacteria, but NOT all viruses; their diameter can vary between 0.3 microns and, for example, 0.04 microns for hepatitis and 0.02 microns for winter vomiting/norovirus.

It is important to bear in mind that this type of filter (as Colin mentions above) certainly won’t remove chemical contaminants, for example, pesticides, herbicides, nitrates, etc.

A few filters have much smaller pores, for example the Sawyer PointONE or the MSR Guardian. You will be able to spot these easily because they specifically state that they remove viruses, a subject not mentioned by the filters that don’t.

UV and boiling will kill all bacteria and viruses, but they each have advantages and disadvantages, as has been discussed here at length.

Happy tummies!