noun. A central point or focus of infection by bacteria or other pathogens.

noun. A point or place at which something originates, accumulates, or develops, such as the center around which a calculus forms.

noun. A nest for insects or small animals

noun. A locus of infection in an organism

noun. An originating point for a phenomenon

"Tyndall had shown that in the moving particles of fine dust discovered by a ray of light in a dark room the germs of low forms of life, which would cause putrefaction, were ever present, and ready to spring into life when a favorable "nidus" for the development of the organism was provided."