Apple’s shares were down by 4.8% to $81.29 in morning trading on Thursday as a consequence of Steve Jobs’ announcement regarding his medical leave until the end of June.

He also sought to assure employees and investors that he will keep a hand in Apple's business while on leave, and planned to remain involved in major strategic decisions. Chief operating officer Tim Cook will stand in while Mr. Jobs is away. Cook, who joined Apple in 1998 as senior vice president of operations, also took over for Jobs for about a month in 2004 when Jobs was undergoing cancer treatments.

In a letter to employees on Wednesday, the Apple co-founder said he would take himself "out of the limelight" for six months after learning in the past week that his still vaguely defined "health issues" are "more complex than I originally thought."

Jobs statement came less than two weeks after he said that he wouldn't discuss his health anymore. On January 5, he announced he was undergoing treatment for a “hormone imbalance" that was responsible for the dramatic weight loss he experienced over the past year.

As far as Steve Jobs’ health history is concerned, there was the pancreatic cancer back in 2004 that no one knew about until Jobs declared it over; the strange and sudden weight loss; the ongoing rumors of a recurrence of cancer; the hormone imbalance that just last week Jobs declared was not going to hurt his ability to run the company.

Five years ago, the Apple co-founder was diagnosed with a very rare form of pancreatic cancer called an islet cell neuroendocrine tumor, which represents about 1% of the total cases of pancreatic cancer diagnosed each year. The disease is curable by surgical removal if properly diagnosed from the onset.

While the top executive's health would be an issue for investors in any company, at Apple the level of concern has been heightened, because Mr. Jobs is involved in everything from ideas for new products to the way they are marketed. He co-founded Apple with Steve Wozniak in 1976 at the beginning of the personal computer revolution. He left the firm in 1985 but returned in 1997 and became full-time chief executive once again in 2000.

In his tenure, Jobs has provided a clear voice and direction regarding markets that Apple would enter and the types of products, such as the iPhone and the iPod, for those markets. It remains to be seen whether his disdain for consensus-building and the homogenization of ideas will be discarded in his absence.

But Cross Research analyst Shannon Cross said Cook can handle the CEO role and that Apple has in recent years hired well in its executive ranks. Nonetheless, analysts say the lack of clarity over who will hold the reins in coming years could affect the company in many ways, including whether it can keep coming out with popular products like the iPod and iPhone.