If you want to see something really scary, go to the Web site of the Centers for Disease Control and Prevention in Atlanta, which has been tracking the rise in obesity. You can see the obesity epidemic spreading like cancer, metastasizing across the country from 1985-2005. It looks as though an alien force or a conquering army is taking over the United States, state by state, year by year.

Almost two-thirds of adults are overweight (body mass index, or BMI, between 25 and 30) or obese (BMI over 30). (To calculate your BMI, click here for adults, here for kids.) Worse, a study in the Annals of Internal Medicine that followed 4,000 people over 30 years found that nine out of 10 men and seven out of 10 women will eventually become overweight. And it's not just adults. Since 1970 the percentage of kids who are overweight or obese has risen almost fourfold, from 4.2 percent to 15.3 percent. New CDC figures released today and published in the Journal of the American Medical Association suggest that the rise in childhood obesity may have leveled off—the latest numbers are approximately the same as the last time the survey was done—but it's not clear yet whether the upward trend has been permanently stalled or whether it is just a statistical artifact.

According to the U.S. Department of Health and Human Services, obesity may account for 300,000 premature deaths a year, almost as many as deaths from cigarette smoking. People who are obese have a 50 to 100 percent increased risk of premature death from all causes compared to those who are not overweight, including heart disease, diabetes, high blood pressure, gallbladder disease, sleep apnea, osteoarthritis and some cancers.

Even though today's numbers offer some hope, it's much too early to assume that the problem has been solved—this may still be the first generation in which kids have shorter lifespans than their parents. According to former U.S. surgeon general Richard Carmona, "As we look to the future and where childhood obesity will be in 20 years … it is every bit as threatening to us as is the terrorist threat we face today. It is the terrorist threat from within."

Well, it doesn't have to be this way. Childhood obesity is almost completely preventable. We don't have to wait for a new drug or technology; we just have to put into practice what we already know. Clearly, genes have changed little, if at all, in the past 40 years. What's changed is our diet and lifestyle. If we caused it, we can reverse it.

Dr. Risa Lavizzo-Mourey decided to do something about it. Under her leadership as president and CEO of the Robert Wood Johnson Foundation, the group has committed $500 million to reverse the rise in childhood obesity. I spoke with Lavizzo-Mourey recently, after she gave the chancellor's health policy lecture at the University of California at San Francisco. Excerpts: