Suing the NFL for Head Injuries

About 700 former NFL players are suing the National Football League over concussion-related injuries. I don't really understand the concept of suing the NFL for incurring repeated concussions but after reading the article I gained a little insight.

Apparently, the plaintiffs say the NFL was aware that players risked permanent brain damage if they played too soon after a concussion but did not inform the players or stop them from returning to play.

The plaintiffs want punitive damages, financial compensation, and payment for medical treatment but they would also like the league to take care of all former NFL players regardless of whether they were listed as plaintiffs on the lawsuit.

It's an interesting case---how can the players prove the NFL knew the risks?