Today, medical researchers at Boston University are publishing the most extensive study ever undertaken of the brains of pro athletes. The results are as grim as you expect, concluding that 69 out of the 85 brains were afflicted with CTE. 

High school football player Nathan Stiles—who died in 2010 after bleeding occurred in his brain during a football game—had early signs of CTE in his brain when it was autopsied as part of the study. The 17-year-old didn't die from that condition but from the brain bleed, likely caused by an earlier concussion that hadn't had time to fully heal before Stiles was tackled in subsequent games.