For all the research that has been done on the human brain over the last few decades, we still know very little about how a lot of conditions happen.
A number of mental illnesses have come to light, including clinical depression, bipolar disorder, borderline personality disorder, and of course, post-traumatic stress disorder (or PTSD). These are conditions experienced by literally millions of people worldwide, but we still have a lot to learn about how these conditions develop within the people who suffer from them.
PTSD is one that has taken literal decades for us to understand even as well as we do know. First recorded as "shell-shock" during the First World War, it was primarily recognized in soldiers returning home from the frontlines, whose experiences when facing death on a mass-scale had changed them mentally.
However, as the condition was revealed to have much more to do with personal trauma, we've come to find out that plenty of non-combat traumatic situations can cause forms of PTSD as well. Most significantly, researchers have made something of a huge breakthrough; you may actually be able to inherit PTSD from your ancestors.