They're called invisible war wounds. And you've probably heard about one of them: the devastating psychological condition called PTSD, post-traumatic stress disorder.
It's called a traumatic brain injury, and it can affect a person's memory, learning, even their personality.
A Chicago-area civilian has become the first brain injury patient to write a book about TBI, as it's called. Brian Sweeney was assaulted 17 years ago, kicked in the head while he was unconscious. He almost died.
Brian still has to cope with memory loss, emotional episodes, and a brain that often races from thought to thought. But he's come a long way.
In "Every 21 Seconds," Brian said he wants to help other families understand what to expect on the road back from a traumatic brain injury, and to understand what they can do to help.