Midian is a dark fantasy game. Most churches in Midian are moral belief structures & guidelines rather than a system that contains a deific figurehead (or many such) and an afterlife.
Asclepius' flies are similar in size to the ordinary housefly, but they are white, with crimson eyes (ugly little creatures). Unlike most flies, however, they are not diseased, in fact their remarkable immune system contains agents which tackle even human illnesses. This is the source of their white colouring also. The standard technique for capturing them, to use their juices, is to tempt one onto the palm of one's hand and then to quickly wring your palms, then rub the mush onto the afflicted area. This is not for the squeamish, but has definite healing possibilities.