Webster's dictionary defines an epidemic as a widely spreading disease that effects a large portion of a community. By any criteria you use, obesity is widely spreading. Here are some facts:
Obesity rates in the United States are up 50% in the last 10 years
Morbidly Obesity (BMI > 40) rates in the United States are up 400% in the last 10 years
Childhood Obesity rates in the United States are up 400% in the last 40 years
Adult diabetes occurring in the youth (MODY) rates in the United States are up 600% in the last 10 years
65% of adults in the United States are now overweight or obese
In the year 2000, for the first time in history, more people worldwide were overweight than underweight
In the next 10 years, 500 million more people worldwide will become overweight according the World Health Organization
Direct costs attributable to obesity are over $115 billion dollars a year in the United States alone.