Attack of the Bucolic Plague

RAY: This puzzler came to us a while ago-- January 1999, to be precise. It's from Professor Bruce Robinson at the University of Tennessee in Knoxville.

Of course, I had to make a few modifications...

TOM: He won't even want to be associated with it, once you're finished.

RAY: I'm sure he'll send us an e-mail asking to have his name expunged.

Here it is:

"A dreaded new disease is sweeping across the countryside. It's called 'The Bucolic Plague.' If you're afflicted with it, you begin wandering around the woods aimlessly, until you finally collapse and die. The remedy is to lock yourself in the bathroom for two or three days, until the urge passes.

"A test has been developed that can detect whether you have the disease. The test is 99 percent accurate. That is, if you have the disease, there is a 99 percent chance that the test will detect it. If you don't have the disease, the test will be 99 percent accurate in saying that you don't.

"In the general population, 0.1 percent of the people have the disease-- that's one-tenth of one percent.

"You decide to go for the test. You get your results: positive."

Should you lock yourself in the bathroom and ask for a constant supply of magazines, or should you not be worried?

And, the real question is, what is the probability that you actually have the Bucolic Plague?

Think you know? Drop Ray a note!

[ Car Talk Puzzler ]