Do you need to filter and/or purify water from UK mountain becks and burns? At one time one would not have bothered, but these days, especially in areas that the internet has made popular for "wild camping", it would seem to be a sensible precaution.
Abroad it is a different matter, where in many countries, particularly in the hotter parts of the world, water-borne parasitic and other diseases are so common that you would be mad not to take precautions.
When it comes to filters, one needs to distinguish between those that remove bacteria (eg E.coli, Salmonella), protozoa (Giardia, Cryptosporidium) and silt, but not viruses, and those that deal with the latter as well - these are generally bulkier and more expensive, but fortunately not usually considered necessary in the UK.
When it comes to trying not to infect others, the rules are simple: go well away from any water source and bury or carry out all faecal waste. If camping in areas where washing your hands will be difficult, a small container of hand sanitiser is worth having.
On this page we group together products useful in preventing yourself and others from becoming infected with any nasties.