Parasitism - A close relationship between two organisms in which one organism, the parasite, benefits at the expense of the host organism.⏎
Pathogen - A biological agent that causes disease.⏎
Peptic Ulcer - An open sore that develops on the inner lining of the stomach or upper small intestine.⏎
Natural Selection - The process by which certain heritable traits that contribute to the survival and reproductive success of an organism become more widespread within a population over successive generations.⏎