But don't freak out just yet: Not all scientists are buying this theory.
The virus, called Ad36, is what's known as an adenovirus. Colds, flu, encephalitis, meningitis, and some cases of diarrhea are caused by adenoviruses, explains Richard Atkinson, MD, emeritus professor of medicine and nutrition at the University of Wisconsin at Madison.
"There are some 36 viruses that cause upper respiratory infections, some cause gastrointestinal problems, some cause inflammation of the brain," Atkinson tells WebMD.
Viruses can damage an organ, then disappear, he explains. "The polio virus leaves nerves damaged. Several viruses cause obesity in animals by causing inflammation and damaging the central nervous system. We don't think our virus does that."
Atkinson has led several investigations of Ad36 -- studies involving animals and humans. Published studies have shown that when the virus is given to chickens, mice, monkeys, and rats, their body fat increases by 50%-100% even though they ate the same amount as animals not given the virus.
"The virus seems to change body composition, so there is an increased percentage of fat," says Atkinson. "In monkeys, weight gain was about four times the weight gain of [comparison monkeys]." But there does appear to be some positive affect. Total cholesterol and LDL "bad" cholesterol levels go down, he says.
At two recent scientific conferences, he presented data on a study conducted in three U.S. cities involving more than 500 obese and normal-weight people who had their blood tested for antibodies to the Ad36 virus.
The results: About 30% of obese people had antibodies to this virus, compared with just 10% of normal-weight people. "If you have antibodies to the virus, you have been exposed. People testing antibody-positive were quite significantly heavier," Atkinson tells WebMD. This study will appear in an upcoming issue of the International Journal of Obesity.
If his results are true, then the virus could be part of this worldwide epidemic of obesity, says Atkinson. "It's reasonable that this virus is a contributing factor. ... Virtually everyone who gets this virus gains weight. It's a pretty robust phenomenon."
There are no obvious symptoms of this virus infection. "Whatever disease [the virus] causes, it's certainly a very mild disease," he explains. "We've inoculated four species of animals and have never lost an animal. The monkeys maybe looked under the weather for a day, then they were bouncing around, they were fine."
Atkinson's new company, called Obetech, will soon start offering blood tests for this virus.
"Obese people suffer a huge amount of discrimination," says Atkinson. "They're told they don't have willpower, they're weak, with no self-discipline. I've been with people who tell fat jokes. I tell them I'm looking for volunteers -- that I want to squirt this virus up their nose, that it will likely cause them to get fat. That generally shuts them up."
One more reason people might want this test: Obesity treatment is not covered by insurance. He says he's going to lobby to get Medicaid, Medicare, and insurance companies to cover it. "If this test can show that some obese people are fat because of a virus, it's going to be hard not to justify covering their care," Atkinson tells WebMD.
Blame Lifestyle, Not Virus
Rudolph L. Leibel, MD, is a molecular genetics researcher in the Naomi Berrie Diabetes Center at Columbia University in New York. He calls the fat virus a theory -- not a public health warning. "My educated guess is this is not going to become a major health issue," he tells WebMD.
Bad lifestyle is causing the obesity epidemic -- overeating, drinking too many sodas, not getting enough exercise, Leibel says.
"There are viruses that will infect the central nervous system and cause injury to specific cells within the brain," he tells WebMD.
Infections can cause animals to become obese, probably by injuring the hypothalamus, Leibel adds. "But that occurs only sporadically."
The idea of infectious obesity is "interesting ... plausible, but extremely unlikely," he tells WebMD. "You would have very mild meningitis or encephalitis, a very low-grade infection. I'm sure it does occur, but I don't think it occurs with a frequency that would cause an epidemic."
But Leibel holds the door open for the flu virus theory. "There are ideas in science that seem very, very radical at the time they are proposed. They are dismissed, but then later on proven to be true. Before germs were discovered, people had lots of explanations for diseases. Before Copernicus, people had lots of ideas about the solar system."