A referee died in the mid-1300s
Refereeing a Quidditch match was once a task for only the bravest
witches and wizards. Zacharias Mumps tells us that a Norfolk referee
called Cyprian Youdle died during a friendly match between local
wizards in 1357.
Quidditch Through the Ages
Harry notes that Quidditch-related deaths (among players) are vanishingly rare
Harry learnt that there were seven hundred ways of committing a
Quidditch foul and that all of them had happened during a World Cup
match in 1473; that Seekers were usually the smallest and fastest
players and that most serious Quidditch accidents seemed to happen to
them; that although people rarely died playing Quidditch, referees had
been known to vanish and turn up months later in the Sahara Desert.
Harry Potter and the Philosopher's Stone - Chapter 11: Quidditch
With reference to injuries, on at least one occasion a wizard is subject to an impact that would have probably killed a muggle. Trained mediwizards are on hand to ensure that he makes a full recovery
‘It’s time out!’ yelled Bagman’s voice. ‘As trained mediwizards hurry onto the pitch to examine Aidan Lynch!’
‘He’ll be OK, he only got ploughed!’ Charlie said reassuringly to Ginny...
Harry Potter and the Goblet of Fire - Chapter 8: The Quidditch World Cup