Intermediary
A mediator jammed in the middle of a group of angry people ends up becoming the intermediary between them. The mediator can't just ignore what's happening around them, and they have the tools to fix this.
ばいかい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
鳥が伝染病の媒介となると仮定しよう。
Let’s assume birds have become an intermediary for infectious diseases.