This is fairly typical behavior for most cats who have been abandonded at some point in their lives. It is a dominance issue of sorts- hard to explain on a bulletin board. There is an excellent book by Dr. Nicholas Dodman that addresses this very behavior. It is called The Cat Who Cried for Help. I cannot recommend it highly enough. He gives you detailed instructions on how to stop this behavior as well as suggesting some things you might be unintentionally doing to fuel this behavior- through no fault of your own.
Also if you follow the link- you will find a listing for a Cat Behavior Hotline that will help you as well. You want to be careful with any cat bite or scratch and clean it as best you can with betadine scrub and apply antibiotic creme to it as well. I had one kitty that used to do this, and I started clipping a long piece of string to my belt loop so that it trailed behind me when I walked through the house. Chester ended up concentrating on the string and not on me. Eventually, working with him through the exercises in the book he was calmed out of the behavior and re-homed to a loving family. He was a feral who had been abandoned. Good luck!Meowhoo