<Dd> Modern history of American football </Dd> <P> Close relations: </P> <Dl> <Dd> Medieval football </Dd> <Dd> Old division football </Dd> <Dd> Rugby football </Dd> <Dd> Association football </Dd> <Dd> Canadian football </Dd> </Dl> <Dd> Old division football </Dd>

What is the number of concussions on football