Short answers, pulled from the story.
Slayton was medically disqualified on the 15th of March 1962, because of idiopathic paroxysmal atrial fibrillation, an irregular heart rhythm with no identified cause. He had been detected during a centrifuge training session in 1959. He was removed from his scheduled mission, Mercury-Atlas 7, and later barred from all remaining Mercury flights.
Slayton flew in space on the 15th of July 1975, as the docking module pilot of the Apollo-Soyuz Test Project. He was 51 years old, making him the oldest person to fly in space at that time. NASA had returned him to flight status on the 13th of March 1972, after examination at the Mayo Clinic.
As a test pilot, Slayton shared the first name Don with another pilot in his unit. To avoid confusion during radio communications, controllers referred to him by his initials, D.K., which gradually shortened to Deke.
As director of Flight Crew Operations, Slayton made crew assignments for the Gemini and Apollo programs. He created a rotation system where backup crews became prime crews three missions later, a schedule that placed Neil Armstrong, Buzz Aldrin, and Michael Collins on Apollo 11. He also replaced Ken Mattingly with Jack Swigert on Apollo 13 over rubella concerns.
Slayton formally left NASA on the 27th of February 1982, after logging 7,164 flight hours. He became president of Space Services Inc. in Houston and served as mission director for the Conestoga rocket, which on the 9th of September 1982, became the first privately funded rocket to reach space.
Slayton was inducted into the U.S. Astronaut Hall of Fame on the 11th of May 1990. The main road through League City, Texas, FM 518, was renamed Deke Slayton Highway. The Deke Slayton Memorial Space and Bicycle Museum in Sparta, Wisconsin, holds his Mercury space suit and his Ambassador of Exploration Award containing a lunar sample.