One of the four major groups of microorganisms (bacteria, viruses, fungi, and parasites) that may live freely in nature, live on another organism without harming it, or live at the expense of the host organism
1) a hollow space or fluid-filled cavity surrounded by a distinct lining that develops abnormally in tissue or in an organ in the body, such as "ovarian cyst" 2) a cellular form of a parasite that has a thick cell wall, which allows for survival of the parasite in the environment and transmission into an uninfected host