This sneaky killer often complicates recent surgery or prolonged immobilization.

Stagnant blood sludges into clots in large veins in the legs or pelvis, and then washes along with the blood flow through the right side of the heart and splatters into the tiny blood vessels inside the lungs -- like slugs building up in your swimming pool filter.

The blood clots -- called emboli -- are then gradually broken down or consolidated into scars that can interfere with your plans to become a professional hockey player, make it hard to walk to the mailbox to get your social security check -- or even, well, never mind.

Symptoms of breathlessness, pain with inspiration or rapid heart rate can be easily misdiagnosed -- a deadly mistake for some. On the other hand, the best way to diagnose the problem requires an expensive CT-scan, which involves the hazards of intravenous dye and radiation that can raise your long-term cancer risk.  If the diagnosis is positive, treatment requires months or even years of strong anticoagulants that carry additional dangers from life-threatening hemorrhage.

How can you avoid a pulmonary embolus?  Stay active! Follow your surgeon's advise of keeping injured extremeties slightly elevated and cooperating with physical therapy.  Above all, avoid sitting idly in front of a computer screen, reading stuff like this.

True hypochondriacs are terrified by tender veins in their arms, or bruises or bumps on their legs, but in most cases these pose no threats.

For something more likely to worry you, try the next diagnosis.