Concussion is traumatic injury that can happen with a blow to the head as the brain moves inside the hard skull.
There can be short-term headaches, blurred vision, vomiting, slurred speech, drowsiness or confusion and memory, balance, mood or sleep problems.
A single concussion will not usually cause permanent damage, but repeated ones over a lifetime can be extremely serious.
Repeated head blows can cause something called chronic traumatic encephalopathy, or CTE, which gets worse over time and can lead to dementia.
Many contact sports, like rugby, have concussion protocols and guidelines to help protect players.
Fifa and the World Health Organization have launched a new “Suspect and Protect” awareness campaign to increase recognition among footballers.
Fifa president Gianni Infantino said: “Concussion is a brain injury and should always be taken seriously.
“By knowing the signs, being aware of the risks and treating concussion correctly, you can help to put player safety first.”
Anyone who has an injury affecting the head, face, neck or body should get a medical check for concussion symptoms, which can take a few days to appear.
One of the Pittsburgh Medical team, Dr Joseph Maroon, co-developed the rigorous concussion checks used by the US National Football League, National Hockey League and Major League Baseball to ensure players are fit to keep playing.
He told BBC News: “In the past, it was thought that you didn’t have a concussion unless you had loss of consciousness. We now know that 90% occur in the absence of loss of consciousness.”
He said slap fighting posed an obvious risk and that guidance was needed.
“The worst possible thing to do in terms of possibly incurring brain damage is to return to a contact sport before the brain has completely recovered… the brain is more susceptible to damage with a lesser force,” he said.
“So for an individual to have experienced concussion and then stand up and get hit again, it’s putting the athlete at increased risk.”
