Steve Jobs’ health is in the news — again. Apple isn’t talking — again. And investors are freaking out — again.

So now’s probably a good time to remind ourselves what we know, and don’t know, about the health of Apple’s CEO.

In 2003 Jobs learned that he had a malignant tumor in his pancreas — a large gland behind the stomach that supplies the body with insulin and digestive enzymes. The most common type of pancreatic cancer — adenocarcinoma — carries a life expectancy of about a year. Jobs was lucky; he had an extremely rare form called an islet cell neuroendocrine tumor that can be treated surgically, without radiation or chemotherapy.

On July 31, 2004,  Steve Jobs underwent a modified Whipple procedure — or pancreatoduodenectomy — that removed large parts of his digestive system and reassembled the remaining parts in a new configuration.

Patients undergoing the procedure typically lose up to 10% of their body weight and may suffer digestive problems for the rest of their lives.

The nature of the surgery was first reported in general terms by Fortune investigative reporter Peter Elkind in a March 5 cover story, (“The trouble with Steve Jobs”) and in some detail on this site on June 13 (“Why does Steve Jobs look so thin“).

It is possible that Jobs’ cancer has returned; that fear, and Apple’s reluctance to discuss the matter during its quarterly earnings conference call this week, helped drive Apple (AAPL) share prices down more than 10% in after-hours trading Monday. Apple shares recovered most of the lost territory on Tuesday and were trading higher again Wednesday, after investors calmed down enough to take a second look at Apple’s record third-quarter earnings.

The fact is, the effects of the Whipple procedure are probably sufficient to explain Jobs’ weight loss, without assuming any recurrence of the original cancer. Wednesday’s New York Times reports that Jobs has assured friends that he remains cancer-free, and that he underwent a second operation earlier this year to address a problem that was contributing to a loss of weight. (One hedge fund hired a private detective to tail Jobs four years ago on his way to hospital appointments to determine out how sick he might be, according to a Wall Street Journal report, and some hedge fund managers are talking about hiring private eyes again.)

Apple insists that Steve Jobs’ health is a “private matter.” But it’s also a matter of public record. Jobs has publicly discussed his bout with cancer, both in a memo to his staff and — quite movingly — in a commencement speech to Stanford University’s class of 2005 (see here).

Apple and Steve Jobs may be forgiven their reluctance to delve deeper into the details of his digestive issues. But a little disclosure on that front might go a long way to calming Wall Street’s rattled nerves.