The Collaborative International Dictionary of English v.0.48:

Nidus \Ni"dus\, n.; pl. nidi. [L. See Nidi, Nest.]
   A nest: a repository for the eggs of birds, insects, etc.; a
   breeding place; esp., the place or substance where parasites
   or the germs of a disease effect lodgment or are developed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nidus
    n 1: a central point or locus of an infection in an organism;
         "the focus of infection" [syn: focus, focal point,
         nidus]
    2: a nest in which spiders or insects deposit their eggs