Why did Steve Jobs take leave from Apple?

Amid speculation about why Steve Jobs took medical leave, experts have said the Apple CEO — a cancer survivor — was likely to be suffering from complications from a 2009 liver transplant or from the recurrence of a tumor.

Meanwhile, Forbes has reported that Jobs secretly went to Switzerland in 2009 seeking treatment for neuroendocrine cancer.

Forbes contributor Dorin Levin writes that in 2009, the company was "putting out incredible devices, but it was unclear why the legendary leader wasn't there."

Jobs, who had surgery for pancreatic cancer in 2004, after which he took a leave of absence to recover, took medical leave again in 2009.

"It wasn't until June 20th, two months after the fact, that the Wall Street Journal uncovered the fact that Jobs had undergone a secret liver transplant at Methodist University Hospital in Memphis, Tennessee."

On Monday, Apple announced the third known absence by Jobs for medical reasons.

In a letter emailed to employees Monday, Jobs said the board of directors granted him the leave so that, "I can focus on my health," but that he would be involved in "major strategic decisions for the company." He will continue to serve as Apple chief executive.

The email said that Tim Cook, the Chief Operating Officer, who recently represented Apple during its co-launch of the iPhone with Verizon Wireless, would be responsible for day-to-day operations.

Levin writes that according to Jerry York, a former Apple director, Jobs took an unpublicized flight to Switzerland to undergo radiological treatment at the University of Basel for neuroendocrine cancer. The treatment was not available in the U.S. at the time.

York died in March 2010, prompting Levin's decision to publish his comments.

Apple (AAPL) shares were trading lower Tuesday as investors start to come to grips with the news of Jobs' absence.