Apple Inc. has apparently decided against disclosing the cause of Steve Jobs' death at the untimely age of 56. But it was no secret that he had been battling a rare form of pancreatic cancer, from about 2004, when he underwent surgery for it.

With a very low survival rate of 4 per cent, pancreatic cancer is the fourth most common cause of cancer death in the U.S., as well as across the world. There are two known types of pancreatic cancer - adenocarcinoma and neuroendocrine tumor. Jobs was affected by the latter, an extremely rare form with a prevalence rate of 35 affected patients per 100,000.

Jobs' tumor was removed in 2004 in a successful procedure. But the malignancy had spread beyond the pancreas and, in 2009, Jobs took another leave of absence to undergo a liver transplant. He took his third leave of absence in January of this year, before resigning as CEO in August.

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, Jobs wrote in a letter to the board of directors Aug. 24. Unfortunately, that day has come.

Jobs was battling immune suppression after the liver transplant, an MSNBC report said, quoting a physician who had not treated him. According to Dr. Timothy Donahue of the UCLA Center for Pancreatic Disease in Los Angeles, most patients who receive liver transplants survive about two years after the surgery.

Jobs had tried treating his tumor with alternative therapy - with changes in diet and lifestyle - after he was diagnosed, a Fortune report of 2008 said. He had also undergone radiation-based hormone treatment in Switzerland in 2009, which isn't available in the U.S.

Another high-profile patient of the same form of cancer, Hollywood actor Patrick Swayze, managed to live for 20 months after his diagnosis, taking advantage of chemotherapy treatments.

Unfortunately, the overall median of survival for most patients is generally only five months, said a TIME report.