Depends entirely on the parasite. Many can be picked up outside from grass after wild animals have left droppings or urinated there.
I would strongly push for screening for e. cuniculi, it's NOT uncommon among rabbits, and has the capability to cause all the symptoms you're describing for your bunny.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry