媒介物

1. Origins, structure, and basic meaning

• Structure: 媒介物, read ばいかいぶつ, combines 媒介, mediation or transmission, with 物, a thing.
• Basic usage: It refers to something that carries or transmits, or to a medium that links otherwise separated parties.

2. Common pitfalls and similar expressions

• Function depends on context
  • In a disease context it is a transmitting agent; in the letter example it is a link between people. One English gloss need not fit both contexts equally.
• The pattern となる
  • 媒介物となった says that something came to serve as an intermediary. It does not imply that it was originally made for that purpose.