Its pretty common knowledge today, if you go camping somewhere near a stream you need to boil any water your gather for a couple minutes to kill any bacteria or parasites that might be living in it.
But was this practice observed by humans before germs theory of disease was a thing?
Which wasn't really widely accepted until the mid to late 1800s.
If people did not practice this, didn't they just constantly get sick?
Sometimes, people boiled their water. As Pliny the Elder noted:
Indeed, it is generally admitted that all water is more wholesome when it has been boiled
(from [Plin. Nat. 31.23](http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D31%3Achapter%3D23; see also chapters 21 and 22)).
More common though was using water of good quality in the first place - if the water is already wholesome enough, it doesn't need to be made "more wholesome".
Other old methods of improving water include filtration, sunlight (providing UV sterilisation), and storage in silver or copper containers (which are anti-bacterial).
Water was also boiled for making various drinks such as tea, barley water, and more. In some cases, these drinks were thought to be good for the health, and if they were alternatives to unclean water, this would have been a correct observation, even if not for the right reasons.