From 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]
From 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