The first scientist to describe living cells as seen through a simple microscope

Antonie van Leeuwenhoek, a dutch merchant and scientist, and was also called the "Father of Microbiology". His developing the microscope actually resulted as his work as a merchant, as he merely wanted to describe the threads of cloth in detail, which in turn lead to him devoting time to creating lenses, to see smaller and smaller objects. Being as how he wasn't exactly a scientist,  he referred to microorganisms he viewed as animalcules (animal molecules). 

A tick on a dog is an example of a parasitic relationship because the tick is living on the dog