Apple co-founder and chief executive Steve Jobs announced he was taking a medical leave of absence 18 months after returning to the company following a liver transplant.

“At my request, the board of directors has granted me a medical leave of absence so I can focus on my health,” Mr Jobs said in a three-paragraph e-mail to Apple employees which did not specify his latest health issues.

Mr Jobs, 55, the Silicon Valley legend behind the Macintosh computer, the iPod, iPhone and iPad, said he would continue as chief executive “and be involved in major strategic decisions for the company”.

He said chief operating officer Tim Cook, who took over when Mr Jobs went on nearly six months of medical leave in January 2009, would be “responsible for all of Apple’s day-to-day operations” during his absence.

“I love Apple so much and hope to be back as soon as I can,” Mr Jobs said. “In the meantime, my family and I would deeply appreciate respect for our privacy.”

Mr Jobs underwent an operation for pancreatic cancer in 2004 and received a liver transplant in Tennessee while on medical leave in 2009. He returned to public view in September 2009 looking gaunt but healthy.

The company’s fortunes have been uniquely linked to Mr Jobs, who returned to the flagging company in 1997 after a 12-year absence and introduced innovative and wildly successful products like the iPod and iPhone.

Britain’s Financial Times last month named Mr Jobs its “Person of the Year” and even US President Barack Obama joined in the plaudits to the chief executive of the Cupertino, California-based gadget-maker.

“We celebrate somebody like a Steve Jobs, who has created two or three different revolutionary products,” Mr Obama told reporters at a White House news conference.

Mr Jobs and Steve Wozniak founded Apple in 1976 and introduced the first Macintosh computer in 1984, along with innov-ations such as the computer mouse.

Mr Jobs left Apple in 1985 after an internal power struggle and started NeXT Computer and Academy Award-winning Pixar, maker of hit animated films such as “Toy Story”.

Since then, Apple has gone from strength to strength, starting with the iMac in 1998, the iPod in 2001, iTunes in 2003, the iPhone in 2007, the App Store in 2008 and the iPad last year.

Key dates in the history of Apple

1976: Steve Jobs and Steve Wozniak unveil the first Apple computer in Palo Alto, California. It consists of little more than a circuit board and costs just under $700.

1977: The Apple II with a one-megahertz processor becomes the first mass-produced computer and an instant hit.

1980: Apple becomes a publicly traded company.

1983: Lisa, the first personal computer featuring a mouse for navigating and desktop icons and folders, is introduced. Its failure is blamed on a daunting price of nearly $10,000.

1984: The Macintosh computer makes its debut. It is affordable and features innovations such as a disk drive, a built-in monitor and a mouse.

1985: Mr Jobs resigns after being stripped of control of Apple in an internal power struggle.

1986: Apple chief executive officer John Sculley becomes company president. Mr Jobs starts computer company NeXT and buys Lucasfilm’s computer graphics division, renaming it Pixar.

1996: Apple buys NeXT and makes Mr Jobs an Apple adviser.

1997: Mr Jobs replaces Gil Amelio as head of Apple. Arch-rival Microsoft invests $150 million in the company.

1998: Mr Jobs revamps Apple’s product line, churning out colorful $1,300 iMac desktop computers designed with monitors and drives in the same casing.

1999: The iBook, marketed as a mobile iMac, is introduced.

2001: Apple launches a portable MP3 digital music player, the iPod, for $399 and opens its first retail store in Palo Alto.

2003: Apple opens online music store iTunes.

2004: Mr Jobs undergoes an operation for pancreatic cancer.

2007: Apple introduces the iPhone, launching the era of the touchscreen smartphone.

2009: Mr Jobs goes on medical leave in January, returning to work in June after undergoing a liver transplant.

2010: Mr Jobs unveils the iPad in January and the touchscreen tablet computer goes on sale in April, becoming a huge hit. Apple surpasses Microsoft in May as the largest US technology company in terms of market value.

2011: Mr Jobs announces on January 17 that he is taking another medical leave of absence for unspecified health reasons.

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