SEATTLE — Apple Inc. CEO Steve Jobs is taking his second medical leave of absence in two years, raising serious questions about his health and the leadership of a company at the forefront of a personal computing revolution.

Apple did not provide any further information about Jobs’ current condition, including whether Jobs is acutely ill, whether the leave is related to his 2009 liver transplant or whether he is at home or in a hospital.

Apple spokesman Steve Dowling referred to the text of Jobs’ note to employees, which was made public Monday.

In the note, Jobs, 55, said he will continue as CEO and be involved in major decisions. As has been the case during past illnesses, Jobs said chief operating officer Tim Cook to be responsible for all day-to-day operations.

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

U.S. stock markets were closed Monday for the Martin Luther King Jr. holiday. Apple’s shares plunged 6.4 percent in trading Frankfurt to 243.50 euros ($323.73).

Apple has a history of extreme secrecy when it comes to the iconic CEO’s health, disclosing major illnesses only after the fact.

The company waited until after Jobs underwent surgery in 2004 to treat a very rare form of pancreatic cancer — an islet cell neuroendocrine tumor — before alerting investors. That type of cancer can be cured if diagnosed early, unlike the deadlier and more common adenocarcinoma.

By 2008, Jobs had lost a noticeable amount of weight, but Apple attributed his gaunt appearance to a "common bug." In January 2009, Jobs issued a statement saying the weight loss was caused by a hormone imbalance, and that the treatment was simple. He backtracked a week later, however, announcing a six-month medical leave. During that time, he received a liver transplant that came to light two months after it was performed.

Few CEOs are considered as instrumental to their companies as Jobs has been to Apple since he returned in 1997 after a 12-year hiatus. With Jobs serving as head showman and demanding elegance in product design, Apple has expanded from a niche computer maker to become the dominant producer of portable music players, a huge player in the cell phone business and the inventor, with the iPad, of a new category of modern tablet computers.

Jobs co-founded Apple with Steve Wozniak in 1976 at the dawn of the personal computer revolution. He was forced from the company in 1985 but returned as CEO in 1997, slashing unprofitable product lines and helping rescue the company from financial ruin.

Some investors fear that without Jobs, Apple would not be able to sustain its growth or its high-end minimalist style, and Apple’s shares have surged and crashed over the years with rumors or news about the CEO’s health.

In 2010, the year that brought the highly anticipated iPad, investors seemed accustomed to Jobs’ extreme thinness and focused instead on the early success of the device.

Investors may also have growing faith in Cook, who is seen as a logical successor to Jobs and who took the reins during the two previous health scares. Cook joined Apple in 1998, and ran the Cupertino, Calif.-based company for two months in 2004 while Jobs battled cancer. His performance won him the promotion to chief operating officer in 2005.

Analysts credit Cook with solving problems that Apple was having with inventory management. And under his direction in 2009, the company kept cranking out well-received products including updated laptops with lower entry-level prices and a faster iPhone with many longed-for features.