Apple boss Steve Jobs has announced that he's taking a six-month medical leave of absence from the company, due to ongoing health issues.

In an email sent to Apple employees (and then released to the media), Jobs said that "in the past week I have learned that my health-related issues are more complex than I originally thought".

In the 14 minutes following the news being made public, Apple's share price fell by 10 per cent, knocking almost $6.4 billion off the company's value. Given that this was after-hours trading, the stock could fall further today when US markets open.

Investors are spooked at the news for one main reason: they fear that Jobs has had a reoccurrence of the pancreatic cancer he suffered in 2004.

When Jobs pulled out of the recent MacWorld show keynote, he blamed a "hormone imbalance", with a subsequent LA Times article pointing out that one of the symptoms of active pancreatic cancer is, yes, hormonal imbalances.

While Jobs has argued that his health is a private matter, investors want to know what happens if he is forced to quit Apple (or, indeed, if he were to die).

COO Tim Cook has been put in day-to-day charge of Apple during Jobs's leave of absence, although it's a bit of a leap to suggest that this means he's the designated successor.

"As CEO, I plan to remain involved in major strategic decisions while I am out," said Jobs in the email, before signing off with a cheery "I look forward to seeing all of you this summer."