There are many dangers in being high in the air in a craft made of wood, canvas, and dope, carrying a load of fuel, powered by what is supposed to be a smooth series of explosions in the engine, and creating other explosions by firing a gun.
One effect of all those explosions is that the engine and the gun can get rather hot.
Although the effect of the flu was supposed to be a state secret for the same reason that nations talk about troop readiness only in positive terms, seeing so many sick and dying would erode anyone's feistiness. It is notable that the Armistice came so soon after the peak of flu deaths.
It is estimated now that 50 to 100 million people (quite a range of uncertainty!) were killed by the flu, which is 5 to 10 times as many as WWI killed. And those deaths came fast. According to Barry, it appears that more than 5 percent of the world's population died, most during a twelve-week period in 1918.