Welcome to giardia or whatever other parasites and bacteria are in natural water sources.
Pretty much all natural surface water, no matter the source, is gonna have stuff in it that can make you sick. Maybe some cramps and diarrhea, some potentially lethal. Any time you drink untreated water it’s a risk no matter the “bro science” about how some is “safe”. Even glacial water has bacteria in it. Just some sources the concentration of bad stuff is going to be low enough that your body can hopefully deal with it without you becoming symptomatic.
Use proper filters and treatments designed for biologically contaminated water, or filter and boil your water before consuming. Stay safe out there!
IDK why, but your comment made me think of a really awful business idea... Immunity building microdose water. Basically you sell and advertise water that has a few parts per billion bacteria to build your immune system.
Will you get sick, maybe? Do we accept any liability... no it says so right on the bottle.
Plenty of animals are riddled with parasites, and early humans absolutely got sick. Think of all the cholera epidemics even in recent history. I’m sure some animals get sick but I’d bet their stomachs are a far harsher environment for bacteria and parasites to survive so it’s less likely for them to be ill.