A special thanks to the California Endowment Health Journalism Fellowships/University of Southern California Annenberg School of Communications for making these stories possible.  http://annenberg.usc.edu/CentersandPrograms/ProfessionalEducation/CalendowFellowships.aspx 

All Americans will be overweight by the year 2048.  Sound like science fiction?  Some say it's science fact.  In addition to diet and exercise, you may soon have a new weapon to fight the battle of the bulge.

Dr. Nikhil Dhurandhar believes a virus called AD-36 could be the reason some people are obese. He first pinpointed the idea twenty years ago, at the University of Bombay, with Dr. S.M. Ajinkya.

"This is a human virus. It really loves human tissue," he said.

The search for the so-called fat virus brought him here -- to a CDC map, tracking national obesity trends. States in light blue have the lowest obesity rates. Those in dark blue and red -- the highest.

"So this is '86, '87, '88, '89, '90, '91, '92," he said. "And this, I don't claim it to be an infectious disease, but if you show this to a person who studies infectious diseases, the person would say this is an infectious disease spreading this way."

But something interesting happened in both monkeys and humans -- while their weight went up, their cholesterol levels went down.

A possible reason? Dhurandhar says AD-36 appears to keep the fat in the fatty tissue -- not in organs or other places where it can cause damage.

"What it does is it prevents that fat from going where it should not be going," he explained.

He says humans and animals with the virus also had better glucose levels. He's still trying to figure out why. And when it comes to making us fatter, Dhurandhar says AD-36 isn't the only microscopic culprit.

"To date there are about 10 different pathogens that have been shown to cause obesity by different groups," he said.

But it doesn't stop with identifying the virus. Dr. Dhurandhar hopes to fight fat with science by developing a vaccine to fight the virus.

"One of the things we would like to do as a long term goal is to prevent weight gain in the first place," he said, "which may be relatively easier than treating obesity which is caused by virus."

He says the vaccine wouldn't be available for at least five years. He must first determine how the virus works -- does it do a hit and run on the body? Or does it stay in the body?

"So we are in the process of understanding that, and once we know that, we will be able to determine how to make a vaccine, at what age to give this vaccine," he said.

Dhurandhar says, right now, there's a blanket approach to treating obesity -- diet and exercise -- which is not always successful.

"If we have treatment that is cause specific, maybe we will have more successful treatment," he explained.

The bottom line is that there are many factors in the obesity equation -- including genetics, metabolism, diet, exercise -- and if you believe Dhurandhar, a handful of microbes, including AD-36.

"If we don't have this thought at the back of our mind that certain infections can cause obesity, we will never go after those, and they will never get discovered, they will never get treated," he said.