Chronic obstructive pulmonary disease (COPD) refers to a progressive group of lung diseases, including chronic bronchitis and emphysema, that make it hard to breathe. Millions of people around the globe die each year because of COPD, and nearly 16 million people in the U.S. have a form of the disease.

The No. 1 cause of COPD in developed countries is tobacco smoking, according to Dr. John Costello, consultant pulmonologist at Mayo Clinic Healthcare in London.

"Tobacco damages the airways. It damages the substance of the lungs, as well, and causes emphysema and is the major cause of COPD," he says.

Instead of lighting up, Dr. Costello hopes people will consider quitting smoking. But it's not the only culprit.

"Environmental smoke can also damage the lungs," he says.

It's more than a cough. And anyone with COPD is at higher risk for other diseases, including lung cancer, heart disease, coronary artery disease.

"And, indeed, at the end stage of the condition from right heart failure because the oxygen level is so low," says Dr. Costello.

Bronchitis and emphysema are two forms of COPD and can occur together. Treatments may help.

For some, an inhaler called a bronchodilator might be used to help with cough and shortness of breath. It works by relaxing constricted airways. If the condition is severe, nebulized bronchodilators — the machine you plug in the wall and put in a rather larger dose — may be needed. Along with medication, another option for those with severe COPD is pulmonary rehabilitation.

"Long-term rehab programs have been very successful in centers that specialize in pulmonary disease," Dr. Costello says.

COPD, in many cases, can be prevented, says Dr. Costello. His advice?

"The strongest possible advice here is to quit smoking, to avoid smoking, to avoid smoke in your environment."