If 25% of people are obese strictly because of the presence of a viral infection, what further percent of obesity is significantly aggravated by having a chronic viral infection in addition to other, likely karmic, causes of being overweight?

We would say that in addition to the approximately 25% of those who are obese primarily through a virus caused condition, a further 55% have a karmic predisposition to be overweight and for whom a chronic virus is aggravating things to make matters worse. So the range is 25-80% of those who are overweight, wholly or partly from presence of chronic viral infection in the body.