Apple boss Steve Jobs is taking a medical leave of absence only 18 months after returning to work after a liver transplant, sparking fears that the CEO is still battling major health issues and leaving many speculating about the future of Apple.

Following Steve Jobs’ announcement, Apple stocks in Frankfurt plummeted. Today is a national holiday in the US, so markets were closed and have not yet registered a reaction.

Steve Jobs, the man behind the Macintosh computer, the iPod, iPhone and iPad, has suffered from pancreatic cancer. Over the last 10 years the Apple maverick has had to reduce his role on health grounds three times.

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Making the announcement, Jobs, 55, said that he would still play a role in major decisions but handed the reigns over to Chief Operating Officer Tim Cook.

“At my request, the board of directors has granted me a medical leave of absence so I can focus on my health,” Jobs wrote. “I love Apple so much and hope to be back as soon as I can.”

“I have great confidence that Tim and the rest of the executive management team will do a terrific job executing the exciting plans we have in place for 2011.”

Jobs continued that he would remain chief executive of Apple and would be “involved in major strategic decisions for the company.”

Steve Jobs underwent an operation for pancreatic cancer in 2004 and received a liver transplant while on medical leave in 2009. Since he returned to work, there have been continued concerns about the Apple head’s health.

US President Barack Obama recently applauded Jobs, saying:

“We celebrate somebody like a Steve Jobs, who has created two or three different revolutionary products.

Jobs founded Apple in 1976 and introduced the first Macintosh computer in 1984. Since then Apple has continued to churn out innovative products such as the iPod, iTunes, iPhone and iPad.

Steve Jobs is so inextricably linked to Apple that doubt will be cast over many of the company’s projects still in the pipeline.

IDC analyst Al Hilwa told the Los Angeles Times: “The company still very much revolves around Steve Jobs. […] He has been involved in such detailed work in everything at the company, you really wonder how they can get by and make decisions without him.”