Rabbits can die if the ivermectin dosage is not correct, so for you home treatment people, please see your veterinarian for this one. Earmites cause rabbits to shake their heads frequently and scratch at their ears. If left untreated a middle ear infection could develop which can cause a problem with their balance. Ivermectin is again the preferred treatment. An internal parasite called coccidia can infect the small intestines.
Symptoms can be loss of appetite to chronic diarrhea and occasionally death.
A rabbit is considered to have diarrhea if the droppings are not firm and round. If the droppings are round but squish when you pick them up, your rabbit has diarrhea. The test for coccidia seems to be the one test that veterinarians routinely perform, but I have yet to have one test positive. It's usually unnecessary, but might help your peace of mind to have your newly acquired rabbit tested for the above parasites. But once you have your rabbit, if you keep his home clean these shouldn't be a problem.