I worry about infection spread, too. But I think (now this is just my guess, as I'm no doctor or anything) that there are certain bacteria, fungi etc. that are only able to thrive in certain parts of the body. For example Chlamydia can spread further up the pelvic area and to the liver, but I don't think it can enter the spinal cord.
Sorry to bring it onto STI's unprovoked and so suddenly.