Final answer:
The Apollo 13 team was broken up three days prior to launch due to astronaut Ken Mattingly's exposure to measles, leading to his replacement by Jack Swigert.
Step-by-step explanation:
The original Apollo 13 team was broken up three days before the scheduled launch when it was discovered that Ken Mattingly, one of the astronauts, had been exposed to measles. He was replaced by his backup, Jack Swigert. This last-minute change was a significant moment in the history of the Apollo 13 mission, which was already drawing public interest and concern due to the inherent risks of space travel. Understandably, this disruption added an extra layer of tension to the mission, which would later become famous for its in-flight crisis that led the crew to abort their lunar landing and safely return to Earth.