Tork died of unknown causes, however he had been diagnosed with a rare form of tongue cancer in 2009. Adenoid cystic carcinoma is a kind of aggressive cancer that most usually affects the salivary glands. It is commonly found in people who are between 40 and 60 years old. Although it can occur elsewhere in the body, such as the lungs, breast, prostate, ovaries, brain, and colon, adenoid cystic carcinoma is more common in the head and neck region. In addition to Tork, other Muppets who have died include Mary Jane Croft (1971), who worked on The Muppet Show and performed with the group until her death from stomach cancer, and Caroll Spinney (2011), who played Gonzo the Great and other Muppets. Spinney passed away due to heart failure caused by pneumonia. He was 77 years old. Tork's cancer was also responsible for his retirement from performing with the Muppets. After several months of treatment, he decided to quit performing because he didn't want to jeopardize the ability of the Muppets to continue without him. In February 2010, just over a year after his diagnosis, Tork announced his retirement from the Muppets to various media outlets.