My karate teacher likes to hold Bruce Lee up as an example. Not just of fist-flying and kick-wielding vengeance but also of the dangers of turning your body into a finely tuned machine. Lee's body was so finely tuned that taking a simple over-the-counter painkiller killed him.
I think there's probably something equivalent at the other end of the spectrum. When a person is constantly under stress and operating at the edge of capacity, it doesn't take much to knock them down and keep them down.
A simple cold/flu has managed to keep me brain-fogged and bed-ridden for four days. I'm still not anywhere near full capacity.
Not as dramatic as Bruce Lee perhaps, but then I also can't knock someone through a wall with a one inch punch.