Chronic traumatic encephalopathy, or CTE, is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes.

The disease has been diagnosed in more than 100 former NFL players and arisen as an existential threat to the United States’ most powerful pro sports league.

“CTE is linked to repetitive brain trauma and has a distinct pathology that can only be diagnosed at autopsy,” says Dr. Ross Zafonte, a principal investigator of the Football Players Health Study at Harvard University and executive vice dean at the University of Missouri.