Americans are getting fatter.

Over the last two decades, the prevalence of obesity (search) among adults in the United States has more than doubled.

It is now the second-leading cause of preventable death in the United States after smoking and experts say it will soon surpass health hazards caused by puffing on butts. Experts say obesity causes an increase risk of diabetes, high cholesterol, high blood pressure, sleep apnea, cancer and other health hazards.

"Obesity is killing 300 thousand people a year, so that's an epidemic, its a serious problem," said Dr. Richard Atkinson, president of the American Obesity Association.

Medical experts have long said obesity is caused by a combination of genetics and overeating. But another theory is gaining momentum in the scientific community -- that certain viruses may be responsible. Scientists say this has been proven in animals but not yet in humans.

The virus theory is controversial and has charged the debate over whether to classify obesity as a disease and how much taxpayers should be paying to treat it.

The National Institutes of Health (search) says it should be considered a disease; the U.S. surgeon general says it should not and argues that obesity is a "condition."

"That's not a disease, that's something that's within the control of everyone. And for us to now shift this over to say ... it's society's responsibility is a real sorry day for this country," said Rick Berman of the Center for Consumer Freedom (search).

Obesity is calculated by a person's Body Mass Index (BMI), a mathematical calculation determined by dividing a person's body weight in kilograms by their height in meters squared.

More information on how to calculate BMI can be found on the American Obesity Association Web site.

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