Why is Ken Mattingly not allowed to go on the mission Who is named as his replacement in APollo 13?

Ken Mattingly was grounded from the Apollo 13 mission due to exposure to German measles, which he had not contracted but could pose a risk to the crew. As a precautionary measure, NASA replaced him with Jack Swigert, who had been a backup for the mission. This decision was made to ensure the health and safety of the astronauts during the critical lunar mission.