What induces interferons?

[Timothy R. Hughes]

This might be an idiotic question, but how exactly are
interferons induced?  I think I've read ten recent 
reviews on interferon's effects, and all of them start
right off by saying "cells infected with virus are
stimulated to sectrete interferons..." without
mentioning how that happens.  I can't find anything
on medline, either, but maybe I don't know the right
search terms.

If someone could just point me in the right direction,
it would really help out a lot.

Thanks,

Tim Hughes