6 Serious Medical Symptoms Some medical symptoms are warnings that you need immediate care. Learn to recognize these six.

That new symptom is troubling: the inexplicable swelling in your calf or the blood in your urine. Could it be serious or even life-threatening?

"Your body flashes signals -- symptoms and signs -- that warn you of potential problems," say Neil Shulman, MD, Jack Birge, MD, and Joon Ahn, MD. The three Georgia-based doctors are the authors of the recently revised book Your Body's Red Light Warning Signals.

Fortunately, many symptoms turn out not to be serious. For example, the majority of headaches stem from stress, eyestrain, lack of sleep, dehydration, caffeine withdrawal, and other mundane causes.

But a sudden, agonizing "thunderclap" headache -- the worst of your life -- could mean bleeding in the brain. Being able to recognize this serious symptom and calling 911 may save your life.

Here are six important flashing signals.