A question about an event in the game [spoilers]

[Crusher]

From what we know Clarence is more than capable of messing with Philip's perception, so the Dr might have not been running or moving at all, it could have been an allucination produced by Clarence. He just made it look like the infected was moving until he saw the Dr under the crate, then he made Philip see the infected in the same place.
Or maybe it's just a story glitch :p