Shell shock
When they were in  Iraq and Afghanistan, approximately 380,000 U.S. troops, about 19% of those deployed, were estimated to have sustained brain injuries from explosive weapons and devices. This prompted the U.S. Defense Advanced Research Projects Agency (DARPA) to open up a $10 million study of the blast effects on the human brain. The study revealed that, while the brain remains initially intact immediately after low level blast effects, the chronic inflammation afterwards is what ultimately leads to many cases of shell shock and PTSD.