Immunity is not what people think and can generally only be built up for tolerating larger numbers of bacteria per given swallow of food or water, specific things like Hep A virus, but certainly not for fancier parasites like giardia and cryptosporidium bugs that have a resistant shell surviving for longer periods in water and out of it, see here the differences
https://www.urmc.rochester.edu/encyclop ... tID=P02019Such parasites have evolved with humans and animals to outsmart their hosts. Bacteria are generally simpler bugs but some like Salmonella can still pack a punch and lay you out.
Up to 30% people who get giardia are infected but don’t show symptoms like diarrhoea.
I have been travelling and backpacking on and off between jobs and in my youth from university for about the last 30 years, 5 years total of which going overland from Bali, South Asia, India, Nepal trekking for total of 8 months on and off on 7 longs treks etc, bush hikes in Oz, Africa, NZ and South Americas. I’ve been plenty sick despite treating water perhaps giardia 8 times, several salmonella infections, often regular travelers diarrhoea just comes with the territory, one gets sick you wait it out, take medication, recover and keep going most times.
In my first youth trip overland from Bali to Thailand after 3 moths my immunity probably got better but in India and Nepal more serious parasites and salmonella started hitting me more often due to the really bad conditions there.
In Aust hiking one good thing is that the rain water tanks and streams while still potentially contaminated there is less of the bugs that came from humans so the chances of disease are generally less compared to other parts of the world where toilets have bad treatment systems or none at all. The most likely thing you will get from a rainwater tank is higher than tap water amounts of normal leaf decaying bacteria but it you come across a tank that has not been cleaned and there is a thick layer in the bottom of decaying leaf sludge and slime of millions of bacteria that can give people who are less immune a short bout of diarrhoea that will go away after vomit and several bowel movements. It won’t be like giardia where it will last for several weeks of serious diarrhoea, cramps and severe eggy burps and swollen stomach. Giardia medicine will cure you overnight so can easily be treated if you are carrying medicine on a hike.
Having said other parts of the world are dodgy you gotta be aware of this one in Aust especially in warmer tropics
http://conditions.health.qld.gov.au/Hea ... id-diseaseIt’s not a bacteria or giardia, something bigger and worse tapeworm parasite, can lodge in parts of your body and associated with livestock and wildlife poo in drinking water living in soils where water passes through and straight into your mouth where untreated.