Leeches are placed on the leg of a patient during a leech therapy session inside a hospital in Srinagar May 2, 2008. For centuries leeches have been used in medicine to cure everything from headaches to gangrene. In the mid-1800's, the blood-sucking creatures became so popular that the species became endangered in Europe. Today, leeches are primarily used as a legitimate treatment that can help heal skin grafts and restore blood circulation. Their primary function is to drain blood.