I think that lab tests are wrong much of the time, because they usually only have one or two specimens to deal with. Because I check my stool about every 2 days, I come across many protozoa. Some are normal, but some are not. And those don't always appear. These things "hook" into tissue deep within the bowel, and only ever come out when they detatch for some reason.
I have also discovered that most parasites come with a bacteria, and it is the bacteria that cause most of the problems. Shroom