I have encountered mushrooms that will live on rotting flesh, I have found mushrooms sprouting from a dead rabbit and another growing from a partialy decomposed squirel. Except for the goulish concept they mushrooms were delightful looking (I have no idea what they were). While I know for certain that fungus will grow on living tissue and most certainly on dead skin, I find it hard to believe that the conditions could be right for any of the mushrooms spoken of here to grow on a living body. The ph is likely different, the nutritional requirements, the fact that even a deficient immune system would have no problem destroying fledgling mycelium even if that mycelium could manage to invade living cells.
While I have seen far too many bizarre things in my life to categoricly deny the possiblity of something like this happeneing, I simply don't see how it could.