system
Given the following post, respond as if you're commenting on Reddit:
human
eli5. Why is natural spring and ground water safe to drink? Don't bacteria live everywhere even after natural filtration?
gpt
You can ingest a fair amount of bacteria and be perfectly fine. Even deadly bacteria and viruses in small enough quantities can be dealt with by your immune system (or even just the other bacteria out competing it). 

Bacteria needs food to live. Water running quickly through a cold sandy stream bed filters out a lot of the food making the bacteria content lower, and relatively safer compared to a stagnant or slow moving water source. The clearer the water, the colder and the faster moving, the more likely it is bacteria levels are going to be lower.

I no longer drink out of streams like we did decades ago. A dead deer up stream or a farm dumping pesticides or an algae bloom could prove a source of bacteria being sent downstream. Filtration is a good idea, or iodine tablets. 

Now, if you are talking about bottled "spring" water, there is a good chance it's pretty heavily filtered to remove bacteria and even viral matter,band pulled from a relatively clean source (underground spring for instance). They will avoid using chlorine to maintain tastes, and really on fine filters and lab testing the water routinely. Most though is just tap water.