WEDNESDAY, Oct. 5 (HealthDay News) -- Steve Jobs, the visionary leader of Apple Inc. who introduced the world to personal computers, then the iPod, the iPhone and the iPad, died on Wednesday following a long battle with cancer.

He was 56.

"Apple has lost a visionary and creative genius, and the world has lost an amazing human being," the company said in a statement it posted on its website Wednesday night. The statement did not cite a specific cause of death.

Jobs announced in August that he was stepping down as head of the hugely successful technology company he co-founded in a northern California garage 35 years ago. The announcement was thin on details, although speculation immediately turned to his ongoing health problems.

He first had surgery for a rare form of pancreatic cancer back in 2004, received a liver transplant in 2009 and took three medical leaves at Apple before turning over the helm to Timothy D. Cook, then chief operating officer, in August. But, even after he left, Jobs was still engaged in the company's affairs, The New York Times reported Wednesday night.

In a letter to Apple's board of directors in August, Jobs said he "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."

This much was known about the health of Jobs, a legendarily private man: Since 2004, he had been fighting a rare form of pancreatic cancer called neuroendocrine cancer.

Although neither Apple nor Jobs' family has given a cause of death, a cancer expert speculated that Jobs' death was due to the neuroendocrine cancer.

"I have never treated him or seen his medical records, but it certainly fits with the treatments that have been acknowledged," said Dr. Steven Libutti, director of the Montefiore Einstein Center for Cancer Care in New York City.

Patients who have had surgery to remove the tumor, but have had the cancer return, often have the cancer come back in the liver. That is probably why Jobs had a liver transplant, Libutti said.

"My speculation is that the tumor returned after the liver transplant and he succumbed to recurrent disease, that would be my bet," he said.

Pancreatic cancer expert Dr. Craig Devoe, from the department of medicine at North Shore-LIJ Health System in New Hyde Park, N.Y., said that "neuroendocrine tumors are uncommon, with only a few thousand cases a year."

For those that affect the pancreas, the numbers are even lower with fewer than 1,000 cases a year in the United States. In contrast, there are around 40,000 cases of other pancreatic cancers a year, Devoe said.

Dr. David M. Levi, a professor of clinical surgery, liver and GI transplantation at the University of Miami Miller School of Medicine, said neuroendocrine cancer "is an unusual tumor. It can arise in a number of places, including the pancreas." Such tumors can also start in the lungs.

It's one of the few tumors that can benefit -- to some extent -- from a transplant, Levi said. Jobs' cancer started in the pancreas and then spread to the liver, making the liver transplant an option, Levi said, adding he has treated patients with this type of cancer and done liver transplants.

While the prognosis for neuroendocrine cancer is often better than for the more common type of pancreatic cancer, in which patients generally live less than a year after diagnosis, neuroendocrine cancer "can also be bad," Levi said.

Neuroendocrine cancer can return after treatment, Levi explained. And while a liver transplant can be effective, "it is not as great a picture as we first thought," he said. "A lot of these patients who have transplants eventually do recur."

"The vast majority of patients that have recurrent disease will die of their disease. One of the problems with the [liver] transplant is that now you are on immunosuppressant drugs, and while they keep you from rejection or destroying the liver, the immune system also would have helped deal with tumors," he explained.

Devoe said a liver transplant is a treatment when "your back's against the wall," and isn't expected to cure neuroendocrine cancer, so very few are done.

"The fact that the disease came back [was] not surprising," he said.

These treatments won't cure the disease, but they may slow its progression, Devoe said. "It may extend the life of patients. But at this point, your best treatments are behind you and survival may be under a year or two," he said. "It's clearly incurable."

Doctors don't really know what causes neuroendocrine tumors. In the past year, the U.S. Food and Drug Administration approved two new drugs for neuroendocrine tumors -- sunitinib and everolimus.

In a commencement address in 2005 to Stanford University graduates, Jobs said: "No one wants to die. Even people who want to go to heaven don't want to die to get there. And yet death is the destination we all share. No one has ever escaped it."

More information

To learn more about neuroendocrine pancreatic tumors, visit Stanford University School of Medicine.