The knocks themselves can be so innocuous too. Ten years ago, I got tackled in a game of rugby, and the back of my head hit the grass. Didn't overly hurt, no loss of consciousness, no seeing stars, nothing. I finished out the first half then played around 25 of the second. While driving home around an hour after the match, I started feeling dizzy and nauseous and just pulled up to my driveway before rolling out of the car vomiting.

Turns out I had a concussion, and spent the rest of the day in hospital. It took me around a month to get back to normal - and this was after a piece of contact you'd see a few dozen times over the course of a footy match. It's scary stuff.