Mold and mildew
Mold spores are always present in the air. They grow when environmental conditions are favorable.
Exposure to extended high humidity or water damage can awaken dormant mold spores.
Fungus growth akin to the type isolated in foxing spots (small brown spots) is unlikely but not impossible when the humidity drops to below 65 percent.
Treating active mold can be complicated; consult a paper conservator. | 
