On August 16, 1920, Cleveland Indians shortstop Ray Chapman was hit in the head by a pitch from New York Yankees pitcher Carl Mays. Bleeding profusely, he walked off the field with some assistance but a mere 12 hours later he died as a result of the injury.
This is the only in-game fatality in the history of Major League Baseball.
The early part of the 20th century is known as the Deadball Era in baseball history, because of low offensive totals and very infrequent home runs. Part of the reason offense was limited was emphasis on defense but also the use of dirty, “doctored” or non-uniform baseballs. Team owners were so tight-fisted that they refused to replace baseballs after they were browned from dirt and tobacco juice unless they had become completely unplayable. This made the balls difficult for the hitters to see as they left the pitcher’s hand.
Sadly, Chapman’s injury showed just how shortsighted the owners’ refusal to swap out fresh baseballs was.
Immediately after the Chapman fatality, Major League Baseball implemented a rule that the umpire would take a ball out of play if it became discolored from dirt (or other modifications) or were in some way cut or imperfect.
(It is believed that as a result of this new rule, and the further outlawing of the spitball, that hitters were given a greater advantage with seeing the ball and offensive totals surged. Of course, this is also when Babe Ruth became a full-time hitter in the big leagues. This rule may have aided in his ascent to baseball immortality.)
In addition to improving visibility of pitches and outlawing the spitball (that would be one odd-looking five-whys analysis) greater cranial protection could have been invented sooner and provided to hitters. However, batting helmets were not mandatory in the majors until 1971.