Most intestinal diseases are highly infectious and are transmitted through fecal waste (that’s poop, dude). The three main disease-carrying groupings according to size are protozoa, bacteria, and viruses. Regarding drinking water in wilderness settings, we’re mostly concerned with giardia and other protozoa because they can survive a long time in water and are a higher risk. Viruses don’t live long in the water so they are a much lesser threat, but should be considered when traveling to other countries.
Man, this is the gross part. You really, really don’t want to let any of these critters set up house in your guts. That is unless you enjoy passing gas, the runs, hurling, bending over and hugging your stomache, and looking like you got run over by a train.
The tough part is that you might not have any problems until you get home, head back to school, and then one day you start feeling just awful. You suspect the lasagne from lunch or the cookies your kid sister made - but it was really that mouthful of water you got when you slipped crossing a stream on a backpacking trip 10 days ago.
Besides that, with Giardia, the symptoms might include loss of appetite for awhile with the diarrhea and then you feel better. But, a couple days later, BAM! it starts over. You might suffer through a few stretches before the symptoms finally stop altogether.