FYI, there are numerous parasitic animals living in us all the time, we don't even notice this. We always get these horror stories about giant tapeworms and things eating us from the inside. But parasitism is just a from of symbiosis. Without symbiosis we wouldn't be able to live. eg. E.Coli bacteria live in out intestine (you drink these in your yakult) and help you digest your food. These bacteria possibly were parasitic too but evolved into mulualistic bacteria wich help us with our every day digestion. Most parasites hardly harm us (don't go dieting with tapeworms though) and on evolutionairy base might even be helpfull. (plus it stimulates natural selection by killing the weak)