A: There may be several such viruses, and yes, you can be tested for antibodies to them. (That would indicate you had been infected at some point.) But since there's no way to reverse the viruses' effect, there's not much you can do with the knowledge other than having one less reason to blame yourself if you're struggling with weight. For the record, I don't think you should blame yourself regardless of what some test may indicate.
Researchers stumbled on this virus–weight gain link by observing that mice, chickens, monkeys, and other animals put on a lot of pounds following an infection with certain viruses, notably adenoviruses. When the researchers tested humans, they found that nearly a third of obese people have antibodies for a particular adenovirus strain, while only 11 percent of normal-weight people do. But the reason there's little hope for treatment is that this is not an ongoing infection: The viruses come and then are beaten back by your body's immune system, but not before, in theory, they've altered your fat cells in a way that increases vulnerability to weight gain—possibly by triggering the cells to grow faster. Some scientists would like to prevent infection with a vaccine. But we are years away from knowing whether such an approach will work. We can be sure that no shot will provide the array of benefits that come from eating well and being physically active. My view is that first we need to solve the aspect of obesity that can be explained by eating too much and doing too little. Then we can explore novel theories—if there is anything left to explain.