From WebMD:

1. If you have unexplained weight loss and/or loss of appetite, you may have a serious underlying medical illness.

2. Slurred speech, paralysis, weakness, tingling, burning pains, numbness, and confusion are signs of a stroke, and you should get to an appropriate emergency center immediately. Early treatment may prevent permanent damage to the brain or even save your life.

3. Black, tarry stools may indicate a hemorrhage from an ulcer of the stomach or the intestine. It is important to stop the bleeding and to rule out cancer as a cause.

4. A headache accompanied by a stiff neck and fever is an indicator of a serious infection called meningitis.

5. A sudden, agonizing headache, more severe than any you have felt before, could mean you are bleeding in the brain. Go to an emergency room immediately.

6. For women: Vaginal bleeding after menopause is a waning sign of possible cancer.

6. For men: A lump in your testicle with or without a small lump in the groin could be serious. Testicular cancer is more commonly found in testicles that did not naturally descend from the abdomen to the scrotum.

Via Lifehacker.com