Well, that seems to be a good question...  (lol)

R.W. Burchfield says, "A person is said to be (a) immune to an infection (i.e., resistant to, protected from or against); or (b) immune from some undesirable factor or circumstance (i.e., exempt from, not subject to). But the division is not clear-cut; in some contexts, from is idiomatically used in type (a) and to in type (b)." All clear?

http://www.dispatch.com/content/stories/life_and_entertainment/2008/11/12/2_KILP1112.ART_ART_11-12-08_D2_K3BRQM9.html