Noun and する verb
ばいかい
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.
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.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.