The phrase "microscopic parasites" refers to very tiny organisms that live on or inside other organisms, using them for food and causing harm to their host. These parasites are so small that they cannot be seen without a microscope. Full definition
It is the army of microscopic parasites injected into your bloodstream that you really need to be concerned about. (newscientist.com)
You should bring a fresh stool sample (deposited in the last 12 hours) with you to the appointment so that the vet can check for microscopic parasite eggs. (dog-health-guide.org)
There are several types of worms and a few microscopic parasites that commonly affect dogs. (pets.georgetown.org)