Why did jack swigert replaced ken mattingly?

Jack Swigert replaced Ken Mattingly on the Apollo 13 mission due to Mattingly's exposure to German measles shortly before the flight. NASA decided to ensure the health and safety of the crew, as Mattingly could have been contagious. Swigert, who was already a backup astronaut, stepped in at the last minute to fill Mattingly's role as the command module pilot.