I have written often here about my own injury due to brain trauma from an automobile accident 34 years ago.
Here is an article regarding brain injury in Ireland. The article states that in Ireland it is believed that as many as 10,000 people every year sustain injury to the brain. Usually the injuries are due to cycling falls without having a helmet.
If that many people are being injured in Ireland, imagine how many are injured in the US. Cycling, car accidents, serious falls, sports injuries can all cause trauma to the brain. Symptoms can show up as late as a few months after the accident. This makes diagnosis hard since doctors often don't tie the symptoms back to the accident. Symptoms can be wide since the brain controls virtually everything going on in the body. Depression, anxiety, insomnia, confusion are just a few of the symptoms that can occur in even mild brain trauma.
I find that people you are chronically ill, often report that their troubles began after an accident of some sort.