Brain Injury more prevalent.

The military has done a great job of recognizing that soldiers surviving concussive injuries are very prone to long term effects.  A TBI (Traumatic Brain Injury) often begins to show up in terms of symptoms within 6 - 8 weeks after an event.  Symptoms most often observed early on are insomnia, anxiety, depression, ADD like symptoms (inattention, emotional outbursts like quick anger, boredom).

This is true in the non-military as well.  Falls and car accidents can also damage the brain.  That proved true for me when after a severe whiplash car accident in college I began to experience symptoms about 6-8 weeks after the accident. 

Here is a good article talking about TBI's and how they are more prevalent than we ever thought.