If you have asthma, chances are you're very familiar with a four-letter word: GERD. An estimated seven out of 10 people with asthma also suffer from gastroesophageal reflux disease -- commonly known as GERD.

GERD is a chronic condition that occurs when the valve that prevents stomach acid from moving back up the food pipe weakens or relaxes, causing a burning sensation in the chest. According to the International Foundation for Functional Gastrointestinal Disorders, GERD affects an estimated 5 to 7 percent of people.

Researchers have studied the link between GERD and asthma for decades with many concluding the two conditions share common triggers. Asthma is a chronic inflammation of the lung's airway tissues affecting an estimated 20 million Americans according to the National Heart, Lung and Blood Institute. Among children, asthma is the leading cause of chronic illness.

Asthma attacks are usually triggered by allergens (a substance your body perceives as a threat) as well as bacterial and viral infections. In response, the airways tighten and fill with mucus. This traps air within the lungs and causes the symptoms associated with an attack, such as shortness of breath, tightness in the chest, coughing and wheezing.

Scientists do not exactly know how the two conditions are connected, but some believe the additional chest pressure that may accompany asthma may cause the valve that closes off the stomach to weaken or relax, allowing stomach acid to back up into the esophagus and throat.

One theory suggests that the acid in the esophagus stimulates nerves that connect to nerves in the lungs, which react by narrowing the breathing passages. It appears to be a cycle that spirals from one kind of chronic reaction to another and back again.

Other studies have found links between GERD and other pulmonary disorders, such as sinusitis and chronic obstructive pulmonary disease (COPD).

The link is not conclusive. Some doctors believe that the pain caused by GERD is perceived as symptoms of an asthma attack and not actually an attack.

A small study, published in the August 2006 edition of the Journal of Pediatrics, cast doubt about whether it was acid reflux or food and liquid regurgitation actually triggering asthma symptoms in children. Scientists monitored 24 children over a 20-hour period and found coughing, wheezing and difficulty breathing accompanied reflux but only in a small number of cases.