(CNN) -- Steve Jobs has been notoriously private about his health, but as he steps down as CEO of the company he helped invent and re-invent, speculation abounds that he has gotten very sick.

In his resignation statement, he alludes to his inability to continue in his role at the company.

"I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know. Unfortunately, that day has come," Jobs wrote in a letter addressed to the Apple Board of Directors and the Apple Community.

Neither Jobs nor the company has revealed concrete details of his condition. Based on the public information about his health history, there are many possibilities, including cancer recurrence and liver transplant rejection.

Steve Jobs: From college dropout to tech visionary

Jobs revealed in 2004 that he had a pancreatic neuroendocrine tumor, kind of pancreatic cancer that tends to grow slowly. It's relatively uncommon, representing only 5% of all pancreatic tumors. Diagnosis usually happens when the cancer has advanced; tumors are often detected after the cancer has spread to the liver, said Dr. Craig Devoe, pancreatic cancer specialist at the North Shore-LIJ Health System in New Hyde Park, New York, who did not treat Jobs.

Treatment options include surgery, hormone replacement, chemotherapy and radiation.
A closer look at Steve Jobs' health

In a rare moment of openness about his condition, Jobs revealed a few details of his diagnosis in a 2005 speech at Stanford University's commencement ceremony.

Jobs recalled having a scan one morning in 2003 that showed a tumor in his pancreas and doctors telling him "this was almost certainly a type of cancer that is incurable and that I should expect to live no longer than three to six months."

"Later that evening, I had a biopsy, where they stuck an endoscope down my throat, through my stomach and into my intestines, put a needle into my pancreas and got a few cells from the tumor. I was sedated, but my wife, who was there, told me that when they viewed the cells under a microscope, the doctors started crying because it turned out to be a very rare form of pancreatic cancer that is curable with surgery. I had the surgery, and I'm fine now."

But the timing wasn't that clear-cut. In between diagnosis and surgery, he pursued an alternative therapy approach for nine months, hoping a special diet would allow him to avoid surgery, according to a 2008 Fortune magazine article.

On July 31, 2004, at the Stanford University Medical Clinic in Palo Alto, California, Jobs underwent surgery to remove his tumor.

"This weekend I underwent a successful surgery to remove a cancerous tumor from my pancreas," Jobs wrote in an e-mail to Apple's staff the next week. "I will be recuperating during the month of August, and expect to return to work in September."

Reports have suggested that this surgery was a Whipple procedure, a complex operation that involves removing the "head" of the pancreas in addition to the gallbladder and parts of the stomach, bile duct and small intestine. This can result in digestive problems and weight loss.

http://www.cnn.com/2011/HEALTH/08/25/steve.jobs.cancer.health/index.html

RIP. . . . . Steve, surely the campus will miss you terribly and the world will miss your great mind. Good nite Steve.