Over the years, I have heard some pretty dodgy advice from people who do not know what they are talking about but this one was repeated to me recently.
In countries or places where the water is not safe to drink, just mix a little bit of dirty water into your clean water, about 10% dirty to clean they say.
This is bad advice.
The most recent person who suggested this claimed to have never had a problem and soon worked up to drinking only dirty water with no ill effects whatsoever.
The key piece of information that these people are missing is that bacteria and protzoa, the causes of most water borne problems are microscopic and the mechanism in which they ¨infect¨ and the severity of the symptoms that they cause are not equal to the amount of them that you ingest.
A Giardia worm does not care if it is alone of if it surrounded by many hundreds of thousands of it´s friends however there is some evidence that suggests that the amount of some parasites in your body are directly proportional to the severity of the symptoms that you receive.
There are few reasons to drink dirty water, building a tolerance to it is not a biologically sound idea though as for this to work, even hypothetically, you will have to first become infected in order for you body to learn how to produce antibodies to the infection in the first place.
This has come up on courses and expeditions that I have run and my advice is always the same. Do not take chances and serious risks with your health through ignorance. Being in blissful ignorance has got most of us through some tight squeezes in life but we, as a civilisation, know enough about how waterborne infections occur that we do not need to take such risks.
A sick person is a liability to a group. In a wilderness expedition, a person who has become sick through an entirely avoidable circumstance put´s other people at risk and can ruin a whole expedition.
There is no tangible benefit to be gained without you paying the price of infection.