When you enter the lower planes you risk infection of the mind. Surrounded by the infernal machinations of a twisted landscape, the human psyche can fracture under the strain of confronting an irredeemably evil universe designed to defile mortal souls and eradicate all traces of hope. Psychological disorders contracted within the lower planes are not always apparent or immediate.
Humans who enter the lower planes are subject to a unique mental illness. Torphrenia.
In some areas of the lower planes simply breathing the air is dangerous. Characters who come into contact with this should save versus breath weapon to avoid contracting the illness. After infection, symptoms don't appear until the character returns to their home plane. The next day they wake up, they will have a sever headache (-1 maximum hit points). Every day that passes the headache gets worse (cumulative -1 maximum hit points). When the character kills somebody (a humanoid) then the pain abates slightly (remove one -1 penalty) and if the humanoid is killed in a particularly painful or gruesome way then the pain almost disappears completely (remove five -1 penalties) but never quite goes away (never drops below -1 maximum hit points). This is not a disease in the usual sense, so typical methods of curing the illness won't work, but attempts to remove a curse or even successful exorcisms will cure the victim.
Humans who physically touch the skin of devils are likely to contract a virulent skin disease. Cubiasis.
At the moment of contact roll 1d20 and add intelligence (add +5 if the character is Lawful), if the total is 24 or greater than the character contracts the disease. Anybody infected with cubiasis can potentially spread the disease through skin on skin contact. Once infected the only way to prevent or cure the disease is either with priestly magic or reducing the character's intelligence to 0 (the latter method make the disease flake off and disappear after one week). The disease is not obvious for three days, symptoms appear on the third day when the infected develops a strange square pattern of reddish lines on their skin. These squares grow out from under the skin like cubes and turn dark red by the fifth day of infection. These red cube-like growths are always tightly packed together and grow it in small patches, slowly overtaking the body. Once the character's skin is completely covered by the growths (after 9+1d6 days, about two weeks) they are overcome by the disease and will look for a quiet, cool place to rest. If they are allowed to rest or sleep, the character dies. Within another day the small cubes will explode into spores and the disease becomes airborne. It is possible to be infected and show no symptoms, evil characters who get infected never show symptoms and are merely contagious.