Why can't Ken Mattingly go on the Apollo 13 mussion?

Ken Mattingly was removed from the Apollo 13 mission due to exposure to German measles. He had not contracted the disease but was at risk of being infected, which posed a potential health threat to the crew. NASA decided to replace him with Jack Swigert to ensure the safety and success of the mission. This decision ultimately became significant, as Mattingly's expertise played a crucial role in resolving the challenges faced during the mission.