My 3 year old bit my newborn leg! very visible bite mark that drew blood on shin can baby get tetanus from this or any other fatal disease??

These wounds can get simple skin or mouth bacteria imbedded that can cause simple infection. Tetanus would be quite far down the list of possibilities. Tetanus only occurs when the tetanus germs cause a local infection after which the bacteria elaborate tetanus toxin. If you clean and sanitize the wound well & apply antibiotic cream I think it will be okay, Call you doc if things bother you.

Keep the area clean and dry, and watch for signs of infection including swelling, redness and drainage. If you have received your Tetanus vaccination during the last trimester, the baby also has antibodies. The human mouth has many germs, so please monitor closely for any changes and let your Pediatrician know about the incident. A local application of Bacitracin is often effective.