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