You might just be possessed. A lot.
Learned from: Insidious (2010)
Unexplained coma? Don't worry, it might not be a medical condition at all—you might just be a possessed. This is the case when it comes to little Dalton from Insidious, who falls into a coma randomly one night, with no apparent cause. As it turns out, there actually is no cause, other than that he's been pulled into a spiritual world called "The Further," and his body is being used as a vessel for malevolent ghosts. Which sucks, of course, but it's better than a real coma, isn't it?
While it's unlikely that this would be the case with your average coma, we would suggest making sure both a priest and a doctor are present regardless, just in case. We knew all those hours of watching horror movies would pay off someday!