Protozoa - A taxonomic group of single-celled microorganisms that live in almost every kind of habitat and include some pathogenic parasites of humans and other animals. ⏎
Parasitism - A close relationship between two organisms in which one organism, the parasite, benefits at the expense of the host organism.⏎
Respiratory Tract - The part of the anatomy that has to do with the passage of air and includes the nose, larynx, trachea, and lungs.⏎
Peptic Ulcer - An open sore that develops on the inner lining of the stomach or upper small intestine.⏎
Pathogen - A biological agent that causes disease.⏎