Vessels. At times a Fallen or one of their servants are caught without adequate time to locate a new body. A vessel is a temporary vehicle where the Fallen can seek refuge for a while. Sometimes in the famous case of Legion, many demons were forced to seek such a temporary refuge. When a human becomes a vessel, the demon or Fallen does not take over or denominate as they do with their long-term mounts, they simply occupy a corner of their mind. Of course this is never healthy for that mind. Nightmares, visions and insanity often are the most typical symptoms.
It is not common for vessels to survive the process. The promptings of the resident demons, the trouble they get him/her into or simply the insanity itself tend to finish them off fairly quickly.