Amoeba time, Giardia 1 to 15 Micron
Protozoa are monads. Till protozoa found a host, they wrap themselves with a membrane (cyst) extremely resistant to environmental influences. It takes about two hours to penetrate these cysts with chemicals. In contrast, the protozoa can be easily filtered out from water due to their size.
Parasitic worms or helminths are eukaryotic parasites, which live inside the host unlike external parasites such as lice and fleas. They are worm-like organisms that live off their hosts. They obtain food and shelter there and lead to disturbance in the nutrient uptake of their host causing weakness and disease. Those who live inside the digestive tract are called intestinal parasites. They can live within humans as well as externally in some animals.