"The appearance of a normal reaction may be just that -- an appearance," he said.
In some unfortunate cases, Faden said the body's natural defense system overcompensates for a brain injury and the result is a cascade of chemical responses that causes more brain damage than the original injury.
"If they over-compensate they can be highly destructive," said Faden "There may be enormous consequences down the line."
In other cases, Faden said the injury to the brain can affect more subtle or "executive" thinking that would take days or months to notice. This means while someone can walk, eat, or do actions that make them appear healthy, very important parts of their thinking and their emotions may be affected.
Dr. Steve Flanagan, of New York University School of Medicine, said the story of railroad worker Phineas Cage has become a textbook example of such a brain injury.
"By some bizarre accident a railroad spike went right though his frontal lobes and he survived," said Flanagan.
Cage could walk, talk, and seemingly acted normal. But Flanagan said the injury left Cage with a new dark, disturbed personality for life even though he did survive.
"A lot of it depends on the caliber of the bullet and the trajectory, how much energy hit the brain," said Flanagan. "Although survival really is exceptionally rare."