It is said that the commonest reason for failure to diagnose pulmonary embolus (blood clot in the lung) is that the treating doctor never considers it. The computer would never forget about pulmonary embolus.

But computers are morons in some ways. They generate a massive list of things a problem could be, but are not very good at differentiating zebras from horses. ("Zebras and horses" refers to a medical maxim: "When you hear hoofbeats, think of horses, not zebras." In other words, because common things occur commonly, don't get carried away with all the esoterica.)

This is where the computer falls down. It can't put your symptoms into context.

Another patient, who actually had abdominal pain, developed some appreciation of the subtleties of diagnosis after he had surfed the Net. He had sharp central abdominal pain, combined with diarrhea and nausea. It didn't seem an exact fit with any diagnosis that I could make, but I wondered about diverticulitis, so I sent him for a CT scan.

I guess the abdominal CT scan has, to a large extent, eliminated the long midline incision that a surgeon would make to allow him to take a look into the abdomen when the diagnosis is "I don't know."

Well, the CT caught us all on the hop; it showed a kidney stone. I don't know that a computer would have done any better.

The computer is the tool, of course, but it is the World Wide Web that you're surfing. There is no shortage of Web sites, but in my investigative search of "red bumps under the arms" I found myself in some pretty bizarre places. I may be a savvy doctor (though some might question that), but I'm no expert on the computer. As noted, WebMD.com got me to HIV. Thedoctorslounge .net offered me a webcam view of a live birth. Another site got me to ingrown hairs, which was halfway relevant. But then there was a link to eBay. Why? In case I wanted to buy some ingrown hairs?

The other problem with the Web is that you almost have to know what's wrong to be able to look it up. For "bumps" you have a choice of pimples, warts, cysts, ganglions, lipomas and a whole lot more. Where do you start?