Pathogen - A biological agent that causes disease.⏎
Parasitism - A close relationship between two organisms in which one organism, the parasite, benefits at the expense of the host organism.⏎
Pandemic - An increase in the occurrence of a particular disease over a very large region, such as a continent or the entire globe, that is greater than what is expected over a given period of time.⏎
Peptic Ulcer - An open sore that develops on the inner lining of the stomach or upper small intestine.⏎