The Mark IX tank was a British armoured fighting vehicle from the First World War. It was the world's first specialized armoured personnel carrier (APC). During the first actions with tanks, it became clear that infantry often could not keep up with the tanks; not because soldiers were too slow—the early tanks themselves could only move at a walking pace—but because soldiers on foot remained vulnerable to machine gun fire, though tanks had been invented to solve that problem. On many occasions, positions gained at great cost were immediately lost for lack of infantry to consolidate. It was thought this problem might be solved by cramming a few infantry soldiers into each tank, but the atmosphere inside was of such poor quality that the soldiers became ill and eventually lost consciousness. Designer M Bergman