What happened if an athlete stumbles in a 100m event and trips the athlete in the adjacent lane too?
Are both of them disqualified or what?
According to IAAF competition rules art 163:
(b) in all races (or any part of races) not run in lanes, an athlete running on a bend, on the outer half of the track as per Rule 162.10, or on any curved part of the diversion from the track for the steeplechase water jump, shall not step or run on or inside the kerb or line marking the applicable border (the inside of the track, the outer half of the track, or any curved part of the diversion from the track for the steeplechase water jump).
So the runner who goes out of his lane will be disqualified, while the impeded runner will not be disqualified.
The impeded runner will probably ask for a re-race or for an automatic qualification and the committee will study the single case.