Foxing is a light brown to rust-colored spotting on aging paper, often identified as being of biologic origin. Once you see it, it is easily recognizable. Many camps believe it is fungal in nature, some (like myself) suspect it to be a bacteria. The brown to rust color is attributed to the oxidation of iron elements in, or on the surface of, aging paper. It is often found in books that have been stored in poor environments, not necessarily high-moisture situations. It is not limited to newsprint or comics, and is found throughout the realm of printed archives.