Q. What is the most common blood type?

The approximate distribution of blood types in the U.S. population is as follows. Distribution may be different for specific racial and ethnic groups:

O-positive 38 percent

O-negative 7 percent

A-positive 34 percent

A-negative 6 percent

B-positive 9 percent

B-negative 2 percent

AB-positive 3 percent

AB-negative 1 percent

Type O-negative blood is called the universal donor type because it is compatible with any blood type. Type AB-positive blood is called the universal recipient type because a person who has it can receive blood of any type.

People over the age of 69 require half of all whole blood and red blood cells transfused, according to the National Blood Data Resource Center (NBDRC).

With an aging population and advances in medical treatments requiring blood transfusions, the demand for blood is increasing. On any given day, an average of 38,000 units of red blood cells are needed.

Q. What exactly is GERD and how can I tell if I have it? I get lots of heartburn.

Heartburn two or more times weekly may be caused by gastroesophageal reflux disease or GERD. See a doctor if you have heartburn too often. The doctor can test for GERD.

In the upper GI series, you swallow a liquid barium mixture. Then a radiologist watches the barium as it travels down your esophagus and into the stomach.

Another test is an endoscopy, in which a small lighted flexible tube is inserted into the esophagus and stomach.

GERD makes stomach acid flow up into your esophagus. There is a valve at the lower end of the esophagus that is designed to keep acid in the stomach. In GERD, the valve relaxes too frequently, which allows acid to reflux, or flow backward.

A hiatal hernia may contribute to GERD. A hiatal hernia occurs when the upper part of the stomach is above the diaphragm, which is the muscle wall separating the stomach from the chest. The diaphragm helps the valve keep acid from coming up into the esophagus.

When GERD is not treated, you can suffer from severe chest pain, narrowing or obstruction of the esophagus, bleeding, or a premalignant change in the lining of the esophagus.

One study showed that patients with chronic, untreated heartburn were at substantially greater risk of developing esophageal cancer.

Q. What is the difference between HIV and AIDS?

HIV (human immunodeficiency virus) is a virus that damages the immune system. This makes you vulnerable to diseases, infections, and cancers. When that happens, you have AIDS (acquired immunodeficiency syndrome), which is the last stage of HIV infection.

HIV symptoms include headache, cough, diarrhea, swollen glands, lack of energy, loss of appetite and weight loss, fevers and sweats, repeated yeast infections, skin rashes, pelvic and abdominal cramps, sores and short-term memory loss.

Your health care provider can test your blood for HIV/AIDS. You can also test your blood at home with the "Home Access Express HIV-1 Test System" that you can buy at your drugstore. It is the only HIV home test system approved by the Food and Drug Administration and sold legally in the United States.

The number of HIV/AIDS cases among older people is growing every year because:

Ÿ Older Americans know less about HIV/AIDS than younger people,

Ÿ Healthcare professionals often do not talk with older people about prevention,

Ÿ Older people are less likely than younger people to talk about their sex lives or drug use with their doctors.

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