Apple's Steve Jobs.
Apple's Steve Jobs. REUTERS

Steve Jobs, co-founder of Apple (AAPL), a visionary, and the driving force behind one of the United States' premiere and most trail-blazing companies in the modern/postmodern era, has had several medical leaves -- one reason Wednesday's decision to step down as Chief Executive Officer has raised questions and concern about his health.

Jobs' recent, public health history is as follows:

In mid-2004, Jobs announced that he had been diagnosed with a cancerous tumor in his pancreas, and said that he had a rare, considerably less-aggressive type known as islet cell neuroendocrine tumor.

Jobs underwent a pancreaticoduodenectomy -- also known as a Whipple procedure -- in July 2004 that appeared to successfully remove the tumor, The New York Times reported.

In early August 2006, Jobs delivered the keynote for Apple's annual Worldwide Developers Conference. His thin, almost gaunt appearance and the delivery was interpreted as without energy or enough vitality.

Two years later, in Jobs' 2008 WWDC keynote address, Apple said Jobs was victim to a common bug and that he was taking antibiotics.

At that time, The New York Times reported, based on an off-the-record phone conversation with Jobs, while his health issues have amounted to a good deal more than a common bug, they weren't life-threatening and he doesn't have a recurrence of cancer.

On Jan. 14, 2009, according to an in an internal Apple memo, Jobs wrote that a week earlier he had learned that my health-related issues are more complex than I originally thought and announced a six-month leave of absence until the end of June 2009 to enable him to better focus on his health. Tim Cook, who had previously acted as CEO in Jobs' 2004 absence, became acting CEO of Apple, with Jobs still involved with major strategic decisions.

In April 2009, Jobs underwent a liver transplant at Methodist University Hospital Transplant Institute in Memphis, Tenn. Jobs' prognosis was determined to be excellent.

On Jan. 17, 2011, one and a half years after Jobs returned from his liver transplant, Apple announced that he had been granted a medical leave of absence. Jobs announced his leave in a letter to employees, stating his decision was made so he could focus on his health.