The Dream Phage:
"The Dream Phage" is an affliction that sometimes displaces creatures of Carcosa into an some place between "The Dreamlands" and the waking world.
In other words this is my cheap quick explanation for why some characters are not present when their player's cannot make a game session. Their character has temporarily taken "ill" with "the Dream Phage".
Such characters "get better" and re-appear somewhere near their companions when the players can re-join the game. This is not always the case. Sometimes it can be assumed that the character was training or otherwise preoccupied somewhere else while the player was absent.
The Shaman character class can temporarily induce "The Dream Phage" in targets.