Steve Jobs, co-founder and CEO of Apple, announced he was taking another medical leave of absence Monday, leading to new speculation about what could be wrong with his health.

No further information about his condition was provided, but this is not the first time Jobs, 55, has taken a medical leave of absence.

In 2004, he took time off to recover from surgery to treat a very rare form of pancreatic cancer — called an islet cell neuroendocrine tumor. In 2009, he took a six-month medical leave from January to June to have a liver transplant.

About 85 to 90 percent of people who have liver transplants will be alive one year later, and about 75 to 85 percent of people will survive at least five years after a transplant, the United Network for Organ Sharing (UNOS) said on its website.

Dr. Alan Benveisty, director of transplant surgery at St. Luke’s Roosevelt Hospital in New York City, said there is a laundry list of reasons as to why Jobs could be taking a medical leave just two years after his transplant, including:

* Infection – Taking immunosuppressant drugs makes a person susceptible to infection, or even cancer;

* Rejection – The body’s immune system can attack the organ and reject it;

* Biliary complications – Such as bile duct problems, where the intestine does not heal, and the bile leaks out or scar tissue blocks the bile duct and bile is unable to flow;

* Medicinal side effects – There are chronic side effects to anti-rejection medications, but Benveisty says it’s unlikely they would cause Jobs to take a medical leave, unless they are affecting other organs, such as the kidney

“All of his original problems could come back,” Benveisty said. “There really could be a myriad of problems… the biggest concern is that his liver is failing or that he has a disease.”

Dr. Nicholas N. Nissen, assistant surgical director of the Multi-Organ Transplant Program at Cedars-Sinai's Liver Disease and Transplant Center in Los Angeles, Calif., and an expert in neuroendocrine tumors, told FoxNews.com that when surgeons do a transplant on a patient with cancer – as in the case with Jobs – they watch that patient very closely.

“When we do a transplant on a patient with cancer, in the next five years, we’re watching for rejection, technical problems, medication problems and cancer recurrence – we’re going to be watching that forever.”

No specific information has been released about the health issues that Jobs may be facing at this time, but Nissen, who has not treated Jobs, said if cancer does come back after a transplant, it can come back in the liver or anywhere in the body.

“When cancer comes back after a liver transplant, there are a variety of treatments, starting with adjusting the immune-suppressing medications,” he said. “You can also add medication directed against the cancer and depending on the location and size of the tumor, you can even consider therapies to try and remove or eradicate the tumors.”

For example, Nissen said if it was an isolated liver tumor, surgeons could remove it or do an ablation on it, which means killing the cancer cells by heating and destroying them.

But, what about another liver transplant?

“We almost never do another transplant because a redo transplant is much more difficult than first-time transplants,” Nissen said.

“And if the cancer has recurred after the first one, it’s most certainly going to occur more quickly after the second one. The fact that the cancer has come back after the first surgery means the transplant wasn’t curative, and so it really doesn’t make sense to do another transplant.”

Patients who receive liver transplants must take immunosuppressant drugs for the rest of their lives to limit their risk for rejection.

But while these drugs serve their purpose, a compromised immune system can leave patients vulnerable to other diseases.

“When you put a patient on life-long medications, you can never stop those medications, because if you do, you can lose the liver to rejection. And we think in general, that cancer re-growth is driven partly by the immunosuppressant medications,” Nissen said.

“You can get yourself into a ‘Catch-22’ situation where the tumor is being driven by the immunosuppressant medications, but you can’t stop them, because if you do you’re going to lose the liver.”

In those situations, Nissen said doctors need to have a few strategies.

“You need to minimize the medications to the lowest acceptable level, so you can try to find other medications or therapies that can work directly on curing the cancer,” he said.”Chemotherapy is also an option as well as radiation, ablation, as well as other targeted agents. Again, it depends on where the tumor is and the size and location.”

Jobs said he will continue as Apple’s CEO and will be involved in all major decisions, but he has asked Tim Cook, the company's chief operating officer to be responsible for day-to-day operations.

FoxNews.com's Jessica Ryen Doyle contributed to this article.