Mechanisms of subsequent infection are less clear. Several hypotheses have been suggested. Nonbacterial inflammation of the fallopian tubes is more common in IUD users than nonusers (22, 74, 277, 339, 440). This inflammation may reduce resistance to disease-causing organisms (95, 339). Some researchers suspect that cervical bacteria can move up the IUD string into the uterus (350, 352). Studies of IUDs without strings generally have not found lower rates of infection, however. One proposed explana- tion for the high rate of infection with the Dalkon Shield is that its multifilament string permitted bacteria to rise into the uterus more readily (296, 336, 372). Strings on all currently available IUDs are monofilament.