IMHO “ghosts” is just an older word vor virus.

People in earlier times knew that some diseases jumped around from person to person and that that could be dangerous, but didn’t have a proper explanation for it.

So they assumed that there must be something invisible in the air that creates the kind of spooky effect that people fall ill sometimes without being touched or physically hit in any way. Kinda spooky, if you think about it and only know about mechanics, but not about cellular biology.