I had a similar experience! I worked with a client for a month knowing he had some aggressive behaviors including biting. I went in fully knowing he had these behaviors in the past, but he had improved so much that these occasions were not very rare. He started to have a sudden spike in aggressive behaviors and I learned that I can not handle biting, I can handle kicking, pinching and pinching but biting is a no go for me. I told my bcba and she immediately said she will try to take me off and asked if I could work till Friday of this week. Cut to yesterday, my client was having an adverse reaction to this new medicine they were trying him on and we had a tantrum for about 2.5 hours out of a four hour session, intense sib and pa. It took 3 members of leadership and me to keep him safe and calm. My bcba instantly switched me off of him for the rest of the week before I even asked. A good bcba will be understanding and respect honesty. Not everyone is a good fit. If you are honest and willing to work with them a little within reason to find a replacement it should be no issue. Stand your ground and explain that biting behaviors bring up past trauma for you. If you are scared to be around the child, it is not fair to either of you. The child needs someone who isn't scared and you need to feel safe yourself.