They are almost never pathogenic for animals. Protozoa are eucaryotes. They are unicellular and usually motile. They do not have true cell walls, rather they have protective covering called a pellicle, or no protective layer outside of the plasma membrane. For examples, amoebic dysentry and malaria.
Helminths are multicellular eucaryotes in the kingdom animalia.
They are microscopic worms. Leeuwenhoek reported his invention of the microscope and discovery of microorganisms to the Royal Society of London.