In the USA 61% of people are now officially classed as obese (with a body mass index, BMI, of over 30) or overweight (BMI of 25-30). The situation is scarcely any better in the UK, with about 20% of people obese and over 40% overweight.
If you look at a map of obesity in the USA, you can watch the epidemic spreading rather like a forest fire from the east coast to the west over the last 20 years. And it is this, says Dhurandhar, which suggests that there could be an infection.