It's mostly a book-collector's problem, and restoration is usually too expensive. But obviously works on paper would have the same problem. I understand that the conservation is to apply dilute bleach - 'foxing' is imprecise, it can be fungal or chemical (iron oxide) and seems to affect paper made in the early 19th century in the states more than other papers.
BTW, I didn't even know what the term meant - the above is from a brief internet search. The most informative link I found was at Octavo:
http://www.octavo.com/collections/marginalia/
foxing.html
