Water is one source, but it seems unlikely to be from a public water supply or bottled. When my dog had giardia about 10 years ago, my vet at the time said she probably picked it up drinking from a rain puddle in the back yard or something. Runoff from a neighbors yard where their dog had it or whatever could transmit it.
Since our gliders don't tend to drink from backyard puddles very often, I've been asking around for a while to find out what other sources could cause it. First and foremost, other gliders and unclean conditions are a prime source. Obviously, glider mills where many gliders are packed into cramped cages which are not properly cleaned are a major source. Giardia and trichomonads are very contagious and care must be taken to avoid contact with an infected glider's bodily fluids or feces. However, my vet said that it doesn't survive well in a dry environment and at least trichomonads apparently don't form a cyst which can survive such conditions. So that still makes me wonder how my new glider picked them up, if she did, from my other glider who had had them months before. Their only contact was swapping pouches which had been checked for feces and being in the same room about 5 feet apart.
I have to say at this point that Piper most certainly did NOT come from a glider mill nor a questionable source. Our own Queenduck or actually her son Chaser owns Piper's parents and I know they take wonderful care of thier gliders. I don't think she's ever even had a sick glider.
Jeepers, Peeper's previous cagemate was a rescue, so she potentially could have been carrying the trichs long term, given them to Peepers and they just proliferated when they got sick. For that matter, Peepers came from a large breeder who I now know has questionable practices. Peeps was my first glider and I got her about 2 years ago.
Another source which has been suggested is fruit. Most of the fruit we buy in our grocery stores comes from South America. When you vacation in Mexico, do you drink the water? That's the same water they are irrigating the fruit with. So it stands to reason that parasites could be living either on or in fruit we cut up for the gliders. I don't know for sure if washing it fully removes them nor do I know if freezing the cut up pieces kills it. So that is a potential source.
Another problem with testing fecals for parasites is that they aren't always "shedding" the parasites so that they can be seen under the microscope. It's been suggested by my vet that they may just show up in the fecals when the glider has diarrhea for other reasons because they get flushed out at that time. Peepers had been symptom free for over a month when I got Piper and Peeps had also had two clean fecals several weeks or maybe a month apart. One was a few weeks before Piper came home and one was the week I got Piper, when I took Piper in for her well check and also had a fecal run on Peepers at the same time. They were both clean at that time, but it was only a week and a half later that Piper started having soft poop and I took a fecal in and it was definitely positive (3+ out of a possible 4 on the grading scale).
So you can see my frustration with these particular bugs. I'm not even sure if they are really what's causing Piper's illness.