If you watch "E.R." or "House" regularly, sooner or later you'll learn what situs inversus is. It's been known to the medical profession since 1643, and is sometimes the focus of professional 'in-house' pranks, when an X-ray is presented to a newbie the wrong way round. In fact, the explosion in cardiac imaging has made reports of subjects with the disorder much more common.

In fact, there are different degrees of this type of misplaced organs. Situs inversus totalis - the left-to-right transposition of all the internal organs, occurs in about one in 8,000 to 10,000 of the population. Sometimes only the heart is on the wrong side of the body. This is known as dextrocardia, and occurs in about one in 29,000.

Occasionally a radiologist, reading hundreds of films a day, may score a case of situs inversus as normal, merely a mislabeled film. This isn't too surprising, as often the condition carries no symptoms.

Unfortunately, some cases of apparent dextrocardia have some organs in the abdomen (e.g. spleen) on the wrong side, too. This is called heterotaxy, and, although rare, it's associated with congenital heart disease, which may occur in a severe form.

Now you know more than you wanted to about mirror-image persons. And you wouldn't notice anything, even when they take their clothes off.