Foxing is a stain due to the presence of ferrrous (iron) particles in paper. It occurs over time under various environmental conditions. It does appear to be biologically sourced, but there is some disagreement as to whether the causal organism is a bacterium or a fungus (or both). It occurs throughout publishing history on all kinds of books, manuscripts, etc., incuding comic books. In comics, it more often occurs on cover stock than newsprint, but this may be due primarily to the obvious exposure of covers to air (oxidation). It is a restoration nightmare.