This article is very informative. The most telling paragraph in my opinion says,

"The authors also point out that the severity of their CTE was tied to how the individuals had played football, which suggests that CTE is more likely to occur as a result of repeated mild injuries, rather than one or two severe ones. The authors stress, in fact, that with CTE, “there may be severe and devastating long-term consequences of repetitive brain trauma that has traditionally been considered only mild.”

http://www.forbes.com/sites/alicegwa...medium=Twitter