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.⏎
Prion - A causative agent of infectious disease that is composed primarily of protein.⏎
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.⏎