One colleague told me, while we were talking about Shelby's health, what he claims is the real story of how Shelby died.
According to this source, who has unparalleled knowledge and whom I absolutely believe, Shelby was at the 2011 SEMA aftermarket show in Las Vegas last November when he happened upon Linda Vaughn, a.k.a. Miss Hurst Golden Shifter, a.k.a. The First Lady Of Motorsports.
Vaughn, as per usual, was showing off her famous décolletage. Shelby loved women almost as much as he loved his cars and went up to Vaughn to give her a hug because, as he apparently said, he just loved "those big old titties."
Probably unknown to Shelby at the time, Vaughn was sick and passed on her bug to him. That sickness landed him in the hospital, where he eventually got pneumonia. He hasn't been in full health since and so, although Linda Vaughn's breasts didn't kill Shelby, it's possible they were the first step.