It's called hypertrophic cardiomyopathy -- a thickening of the heart muscle, which makes it more difficult to pump blood to the body.

According to the American Heart Association, "HCM has been cited as the most common reason for sudden cardiac death in young people and athletes under the age of 35."

Experts say things like "lifestyle changes" can make a big impact for people diagnosed with HCM -- which is obviously why Brown is making the decision to quit football. It's just too risky.

Penn State's head coach, James Franklin, said Brown only discovered the condition while undergoing routine COVID testing -- but noting, "it is not COVID related."

"We learned about this in early September, and we've been working through this and dealing with this as a team," Franklin said.

"The entire organization has rallied behind Journey and his family."

Brown was a force at running back -- earning MVP honors at the 2019 Cotton Bowl by rushing for 202 yards and 2 TDs on 16 carries in Penn State's victory over Memphis.

He led Penn State in rushing in 2019 with 890 yards and 12 TDs.

Brown has posted a statement about the situation -- saying his condition will prevent him from realizing his dreams of playing in the NFL, but "football will always be a part of me."

"The pain of not being able to play the game I love anymore hurts and I can't explain how I am feeling right now. However, I can walk away from the game knowing I truly gave my all at every practice, on every down and in the locker room every day."